Mutators in space: the dynamics of high-mutability clones in a two-patch model.

Clones of bacteria possessing high-mutability rates (or mutators) are being observed in an increasing number of species. In a constant environment most mutations are deleterious, and hence the spontaneous mutation rate is generally low. However, mutators may play an important role in the adaptation of organisms to changing environments. To date, theoretical work has focused on temporal variability in the environment, implicitly assuming that environmental conditions are constant through space. Here, we develop a two-patch model to investigate how spatiotemporal environmental variability and dispersal might influence mutator dynamics. Environmental conditions in each patch fluctuate between two states; the rate of fluctuation varies in each patch at differing phase angles. We find that at low and intermediate rates of fluctuation, an increase in dispersal results in a decrease in the density of mutators. However, at high rates of environmental change, dispersal causes an increase in mutator density. For all frequencies of environmental fluctuation these trends are enhanced as the phase angle approaches 180 degrees. We argue that future work, both empirical and theoretical, is needed to improve our understanding of how spatiotemporal variability impacts on mutator densities and dynamics.

Mutator dynamics in fluctuating environments.

Populations with high mutation rates (mutator clones) are being found in increasing numbers of species, and a clear link is being established between the presence of mutator clones and drug resistance. Mutator clones exist despite the fact that in a constant environment most mutations are deleterious, with the spontaneous mutation rate generally held at a low value. This implies that mutator clones have an important role in the adaptation of organisms to changing environments. Our study examines how mutator dynamics vary according to the frequency of environmental fluctuations. Although recent studies have considered a single environmental switch, here we investigate mutator dynamics in a regularly varying environment, seeking to mimic conditions present, for example, under certain drug or pesticide regimes. Our model provides four significant new insights. First, the results demonstrate that mutators are most prevalent under intermediate rates of environmental change. When the environment oscillates more rapidly, mutators are unable to provide sufficient adaptability to keep pace with the frequent changes in selection pressure and, instead, a population of 'environmental generalists' dominates. Second, our findings reveal that mutator dynamics may be complex, exhibiting limit cycles and chaos. Third, we demonstrate that when each beneficial mutation provides a greater gain in fitness, mutators achieve higher densities in more rapidly fluctuating environments. Fourth, we find that mutators of intermediate strength reach higher densities than very weak or strong mutators.

Interplay between membrane dynamics, diffusion and swelling pressure governs individual vesicular exocytotic events during release of adrenaline by chromaffin cells.

Release of adrenaline by chromaffin cells occurs through a process involving docking and then fusion of a secretory vesicle to the cytoplasmic membrane of the cell. Fusion proceeds in two main stages. The first one leads to the creation of a stable fusion pore passing through the two membranes and which gives a constant release flux of neurotransmitter (pore-release stage). After a few milliseconds, this initial stage which is not investigated here proceeds through a sudden enlargement of the initial pore (full-fusion stage) up to the complete incorporation of the vesicle membrane into that of the cell and total exposure of the initial matrix vesicle core to the extracellular fluid. The precise time-resolved dynamics of the release and of the vesicle membrane during the full-fusion phase can be extracted with a precision never achieved so far by de-convolution of experimental chronoamperometric currents monitored during individual exocytotic secretion events. The peculiar dynamics of the vesicle membrane proves that exocytotic events are powered by the swelling of the matrix polyelectrolyte core of the vesicle, although they are kinetically regulated by diffusion in the matrix and by the dynamics of the vesicle and cell membranes. Two simple theoretical models based on the dynamics of pores are developed to account for these dynamics and are shown to predict behaviors which are essentially identical to the experimental ones. This offers a new view of the kinetic grounds which control the full-fusion stage, and therefore provides a new interpretation of the sudden transition between the pore-release and the full-fusion stages. This transition occurs when the increasing membrane surface tension energy due to the refrained internal swelling pressure overcomes the edge energy of the pore, so that the initial fusion pore becomes unstable and is disrupted. This new view predicts that secretory vesicles which contain matrixes energetically similar to those of the adrenal cells investigated here can be separated into two classes according to their radius and catecholamine content. Small vesicles (less than ca. 25 nm radius, and containing less than ca. 20000 molecules) should always release through pores. Larger vesicles should always end into fusing except if another mechanism closes the pore before ca. 10000 molecules of catecholamines have been released.

Differential quantal release of histamine and 5-hydroxytryptamine from mast cells of vesicular monoamine transporter 2 knockout mice.

The recent availability of mice lacking the neuronal form of the vesicular monoamine transporter 2 (VMAT2) affords the opportunity to study its roles in storage and release. Carbon fiber microelectrodes were used to measure individual secretory events of histamine and 5-hydroxytryptamine (5-HT) from VMAT2-expressing mast cells as a model system for quantal release. VMAT2 is indispensable for monoamine storage because mast cells from homozygous (VMAT2(-/-)) mice, while undergoing granule-cell fusion, do not release monoamines. Cells from heterozygous animals (VMAT2(+/-)) secrete lower amounts of monoamine per granule than cells from wild-type controls. Investigation of corelease of histamine and 5-HT from granules in VMAT2(+/-) cells revealed 5-HT quantal size was reduced more than that of histamine. Thus, although vesicular transport is the limiting factor determining quantal size of 5-HT and histamine release, intragranular association with the heparin matrix also plays a significant role.

Sodium-calcium exchange affects local calcium signal decay and the rate of exocytotic secretion in single chromaffin cells.

The effects of Na+ deprivation on local calcium signal decay and the rate of exocytotic secretion were measured in single bovine chromaffin cells to determine whether Na-Ca exchange influences the local cytosolic Ca2+ signal for neurohormone release. Na+ replacement with N-methylglucamine caused a marked slowing of the decay of the local Ca2+ signal near points of its initiation, as measured by high-resolution fluorescent Ca2+ imaging in the confocal laser scanning microscope. Na+ replacement also resulted in a doubling of the rate and magnitude of exocytotic secretion measured in single cells by high-resolution microamperometry. Release rates provide an independent measure of local active zone Ca2+. Five repetitive stimulations of the same cell in Na+-free, but not in Na+-containing, medium resulted in a progressively increasing rate of catecholamine release, suggesting an increasing level of active zone Ca2+ and a role of Na-Ca exchange activity in Ca2+ clearance between stimulations. As secretory activity and its triggering Ca2+ signals are known to be co-localized in active zones along the plasma membrane, the results suggest that Na-Ca exchange may influence the decay of the local Ca2+ signal for exocytotic secretion. This would be consistent with a contribution to local Ca2+ clearance by a novel mechanism utilizing the insertion of secretory vesicle Na-Ca exchangers into the plasma membrane during exocytosis.

Improving data acquisition for fast-scan cyclic voltammetry.

Described is an improved data acquisition system for fast-scan cyclic voltammetry (FSCV). The system was designed to significantly diminish noise sources that were identified in previously recorded FSCV measurements for the detection of neurotransmitters. Minimized noise is necessary to observe the low concentrations of neurotransmitters that are physiologically important. The system was based on a high-speed, 16-bit AD/DA acquisition board that allowed high scan rates and better resolved the small faradaic currents which remained after background subtraction. Irregularities that occur when independent timing sources are used for generation of the voltage waveform and collection of the current can create large noise artifacts near the voltage limits during FSCV. These were eliminated by the use of a single acquisition board that generated the voltage waveform and collected the current. Noise from frequency drift of the power line was eliminated through the use of a phase-locked loop. To demonstrate the improved performance of the system, data were collected using carbon-fiber microelectrodes in a flow injection analysis system and in brain slices. This new data acquisition system performed significantly better than another system previously used in our laboratory without these features. The improved detection limits of the new system allowed clearly resolved current spikes featuring pre-release "feet" to be recorded adjacent to individual mast cells following chemical stimulation. When combined with false-color plots, the low-noise system facilitated identification of dopamine release in a freely moving animal.

Spatio-temporal resolution of exocytosis from individual cells.

Biophysical events involved in late stages of exocytosis occur at highly localized areas of cells on millisecond and submillisecond time scales. Thus, methodologies with high spatio-temporal resolution are required to achieve measurements at individual secretory cells. Much has been learned about the mechanisms and kinetics of vesicular release through analysis with the carbon fiber microelectrode techniques amperometry and cyclic voltammetry. Coupling of these techniques with other methods such as patch-clamp continues to reveal details of the secretion process. It is now clear that extrusion of the vesicular contents is a more complex process than previously believed. Vesicle-cell fusion, revealed by cell capacitance measurements, is temporally dissociated from secretion measured amperometrically. The stability imparted by interaction and association of vesicle contents at rest results in a rate-limiting extrusion process after full fusion. Furthermore, the presence of partial fusion events and the occurrence of nonquantized release have been revealed with electrochemical tools.

Bacillus thuringiensis Cry1Ac toxin interaction with Manduca sexta aminopeptidase N in a model membrane environment.

The Bacillus thuringiensis Cry1Ac delta-endotoxin was shown to bind in a biphasic manner to Manduca sexta aminopeptidase N (APN) present in a novel model membrane. Surface plasmon resonance analysis allowed the quantification of toxin binding to M. sexta APN in a supported lipid monolayer. The initial binding was rapid and reversible, with an affinity constant of 110 nM. The second phase was slower and resulted in an overall affinity constant of 3.0 nM. Reagents used to disrupt protein-protein interactions did not dissociate the toxin after high-affinity binding was attained. The initial association between Cry1Ac and APN was inhibited by the sugar GalNAc, but the higher-affinity state was resistant to GalNAc-induced dissociation. The results suggest that after binding to M. sexta APN, the Cry1Ac toxin undergoes a rate-limiting step leading to a high-affinity state. A site-directed Cry1Ac mutant, N135Q, exhibited a similar initial binding affinity for APN but did not show the second slower phase. This inability to form an irreversible association with the APN-lipid monolayer helps explain the lack of toxicity of this protein towards M. sexta larvae and its deficient membrane-permeabilizing activity on M. sexta midgut brush border membrane vesicles.

Effects of external osmotic pressure on vesicular secretion from bovine adrenal medullary cells.

Secretion of catecholamines from individual vesicles of bovine adrenal medullary cells was studied with amperometry in media of various osmolarities and compared with results obtained in isotonic physiological buffer (315 mosM). Hypotonic solutions caused an increase in the number of amperometric spikes evoked by brief exposure to 5 mM Ba2+. Under moderate hypertonic conditions (630 mosM), individual vesicular events were decreased in frequency, and lower amounts were secreted per event. Furthermore, the events were temporally broadened relative to those observed during release in isotonic conditions. At 970 mosM, exposure to 5 mM Ba2+ evoked even smaller secretory events that resemble the prespike feature that has been attributed to the initial opening of the fusion pore. The lack of large spikes is not due to failure of Ba2+ entry because fura-2 fluorescence reveals an increase in intracellular divalent ions. After exposure to Ba2+ in hypertonic solution, spikes could be induced with isotonic solution transiently directed onto the cell, but this process was not accompanied by a change in the concentration of intracellular divalent ions. Thus, this procedure provides an unique opportunity to temporally separate exocytotic secretion from entry of divalent ions.

Exocytotic release from individual granules exhibits similar properties at mast and chromaffin cells.

The effects of temperature on granular secretion were studied in individual bovine adrenal chromaffin and rat peritoneal mast cells. It was found that more molecules are released from individual granules at physiological temperature than at room temperature, where such experiments are normally performed. In mast cells, there is also a dramatic decrease in the time required for exocytosis to be complete at 37 degrees C compared to room temperature. In the presence of some cations, the amount released from individual granules at room temperature from both types of cells could be altered. The amount of secretion decreased with the divalent cation zinc but increased with the monovalent cation cesium. These experiments used two electrochemical techniques: cyclic voltammetry and amperometry. With amperometry, the concentration gradient created by the electrode near the cell further increased the amount of release. Similar responses to changes in the extracellular environment in chromaffin and mast cells suggest that the mechanism of extrusion of the granule contents is similar in both cell types.

Microelectrodes for the measurement of catecholamines in biological systems.

Many of the molecules involved in biological signaling processes are easily oxidized and have been monitored by electrochemical methods. Temporal response, spatial considerations, and sensitivity of the electrodes must be optimized for the specific biological application. To monitor exocytosis from single cells in culture, constant potential amperometry offers the best temporal resolution, and a low-noise picoammeter improves the detection limits. Smaller electrodes, with 1-micron diameters, provided spatial resolution sufficient to identify the locations of release sites on the surface of single cells. For the study of neurotransmitter release in vivo, larger cylindrical microelectrodes are advantageous because the secreted molecules come from multiple terminals near the electrode, and the greater amounts lead to a larger signal that emerges from the Johnson noise of the current amplifier. With this approach, dopamine release elicited by two electrical stimulus pulses at 10 Hz was detected with fastscan cyclic voltammetry in vivo. Nafion-coated elliptical electrodes have previously been shown to be incapable of detecting such concentration changes without extensive signal averaging. In addition, we demonstrate that high-pass filtering (200 Hz) of cyclic voltammograms recorded at 300 V/s decreases the background current and digitization noise at these microelectrodes, leading to an improved signal. Also, high-pass filtering discriminated against ascorbic acid, DOPAC, and acidic pH changes, three common interferences in vivo.