Properties and uses of photoreactive caged compounds

Feedback control of inositol trisphosphate signalling bycalcium

Inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ release depends on the cytoplasmic concentration of Ca2+ in a biphasic manner: activated with the increase in Ca2+ up to approximately 300 nM and inhibited by its further increase. Kinetic studies of the Ca2+ release with rapid increase in Ca2+ or InsP3 using respective caged compounds indicated that the effects of Ca2+ appear immediately upon change in the Ca2+ concentration. Recovery from the Ca(2+)-dependent inhibition seemed also rapid after reduction in the Ca2+ concentration. These results indicate that there is a Ca(2+)-mediated positive feedback control of InsP3-induced Ca2+ release below 300 nM Ca2+. This feedback control seems to explain, at least partly, the phenomenon that InsP3 is more effective in the Ca2+ stores with greater Ca2+ loading. The Ca(2+)-mediated feedback control is also expected to give rise to temporally or spatially confined Ca2+ release as well as Ca2+ wave within the cells.

Excitatory signaling in bacterial probed by caged chemoeffectors

Chemotactic excitation responses to caged ligand photorelease of rapidly swimming bacteria that reverse (Vibrio alginolyticus) or tumble (Escherichia coli and Salmonella typhimurium) have been measured by computer. Mutants were used to assess the effects of abnormal motility behavior upon signal processing times and test feasibility of kinetic analyses of the signaling pathway in intact bacteria. N-1-(2-Nitrophenyl)ethoxycarbonyl-L-serine and 2-hydroxyphenyl 1-(2-nitrophenyl) ethyl phosphate were synthesized. These compounds are a 'caged' serine and a 'caged' proton and on flash photolysis release serine and protons and attractant and repellent ligands, respectively, for Tsr, the serine receptor. The product quantum yield for serine was 0.65 (+/- 0.05) and the rate of serine release was proportional to [H+] near-neutrality with a rate constant of 17 s-1 at pH 7.0 and 21 degrees C. The product quantum yield for protons was calculated to be 0.095 on 308-nm irradiation but 0.29 (+/- 0.02) on 300-350-nm irradiation, with proton release occurring at > 10(5) s-1. The pH jumps produced were estimated using pH indicators, the pH-dependent decay of the chromophoric aci-nitro intermediate and bioassays. Receptor deletion mutants did not respond to photorelease of the caged ligands. Population responses occurred without measurable latency. Response times increased with decreased stimulus strength. Physiological or genetic perturbation of motor rotation bias leading to increased tumbling reduced response sensitivity but did not affect response times. Exceptions were found. A CheR-CheB mutant strain had normal motility, but reduced response. A CheZ mutant had tumbly motility, reduced sensitivity, and increased response time to attractant, but a normal repellent response. These observations are consistent with current ideas that motor interactions with a single parameter, namely phosphorylated CheY protein, dictate motor response to both attractant and repellent stimuli. Inverse motility motor mutants with extreme rotation bias exhibited the greatest reduction in response sensitivity but, nevertheless, had normal attractant response times. This implies that control of CheY phosphate concentration rather than motor reactions limits responses to attractants.

Photostimulation using caged glutamate reveals functional circuitry in living brain slices

An approach for high-spatial-resolution mapping of functional circuitry in living mammalian brain slices has been developed. The locations of neurons making functional synaptic connections to a single neuron are revealed by photostimulation of highly restricted areas of the slice (50-100 microns in diameter) while maintaining a whole-cell recording of the neuron of interest. Photostimulation is achieved by bathing brain slices in a molecularly caged form of the neurotransmitter glutamate [L-glutamic acid alpha-(4,5-dimethoxy-2-nitrobenzyl) ester], which is then converted to the active form by brief pulses (< 1 ms in duration) of ultraviolet irradiation. Direct activation of receptors on recorded neurons in rat hippocampus and ferret visual cortex demonstrates that photostimulation is reliable and reproducible and can be repeated at the same site at least 30 times without obvious decrement in neuronal responsiveness. Photostimulation of presynaptic neurons at sites distant to the recorded neuron evoked synaptic responses in hippocampal and cortical cells at distances of up to several millimeters from the recorded neuron. Stimulation of 25-100 distinct presynaptic sites while recording from a single postsynaptic neuron was easily achieved. Caged glutamate-based photostimulation eliminates artifacts and limitations inherent in conventional stimulation methods, including stimulation of axons of passage, desensitization, and poor temporal resolution of "puffer" pipettes, and current artifacts of iontophoretic application. This approach allows detailed physiological investigation and manipulation of the complex intrinsic circuitry of the mammalian brain.

Inositol trisphosphate-mediated Ca2+ influx into Xenopus oocytes triggers Ca2+ liberation from intracellular stores

1. Inositol 1,4,5-trisphosphate (InsP3) functions as a second messenger by liberating Ca2+ from intracellular stores and by promoting influx of extracellular Ca2+. We examined whether Ca2+ influx modulates intracellular Ca2+ liberation in Xenopus oocytes by fluorescence monitoring of cytosolic free Ca2+ together with voltage clamp recording of Ca(2+)-activated Cl- membrane currents. Sustained activation of membrane Ca2+ permeability was induced by intracellular injections of a non-metabolizable InsP3 analogue, 3-deoxy-3-fluoro-D-myo-inositol 1,4,5-trisphosphate (3-F-InsP3), and Ca2+ influx was controlled by applying step changes in membrane potential to alter the driving force for Ca2+ entry. 2. Negative-going potential steps evoked intracellular Ca2+ signals comprising two components; an initial transient peak followed by a slower rise. The initial transient grew steeply over a narrow (ca 40 mV) voltage range but then increased little with further polarization, whereas the second component showed a nearly linear voltage dependence. 3. The transient Ca2+ signal continued to rise almost unchanged when Ca2+ influx was interrupted by stepping the potential to more positive values after brief hyperpolarization. In contrast, Ca2+ levels declined monotonically when positive-going steps were applied after longer intervals during the second component of the Ca2+ signal. 4. Large Ca(2+)-dependent transient inward (T(in)) membrane currents were evoked during the rising phase of the initial Ca2+ signal, but little current was associated with the second component of the Ca2+ signal. 5. The T(in) currents evoked by hyperpolarization were mimicked at fixed clamp potential by re-admitting Ca2+ to the bathing solution, and by flash photolysis of caged Ca2+ loaded into the oocyte. 6. T(in) currents were strongly inhibited by prior release of Ca2+ from InsP3-sensitive intracellular stores, and vice versa. Experiments with paired hyperpolarizing pulses and paired photorelease of InsP3 showed that responses to both stimuli recovered with similar time courses. 7. We conclude that the transient Ca2+ signal and associated T(in) current evoked by hyperpolarization arise because Ca2+ entering the oocyte triggers regenerative release of Ca2+ from InsP3-sensitive intracellular stores. Since membrane currents evoked by liberated Ca2+ were much greater than those evoked by Ca2+ entry per se, a major function of InsP3-mediated Ca2+ entry may be to modulate the activity of intracellular Ca2+ stores.

Transient detection of spin-labeled myosin subfragment 1 conformational states during ATP hydrolysis

We have used time-resolved electron paramagnetic resonance spectroscopy and caged ATP to detect nucleotide-induced changes in the conformational state of spin-labeled myosin heads (IASL-S1). Changes in the internal rotational dynamics of IASL-S1 were monitored with millisecond time resolution during the pre-steady-state phase of ATP hydrolysis. The changes in the internal protein dynamics were rigorously correlated with specific biochemical kinetic transitions, allowing us to observe directly the dynamic sequence of structural changes in IASL-S1 during the binding and hydrolysis of ATP. When caged ATP was photolyzed (producing 500 microM ATP) in the presence of 100 microM IASL-S1, the EPR signal intensity rose transiently to the steady-state ATPase level, indicating increased rotational motion about the SH1 region of the myosin head. Kinetic and spectral analyses have resolved two phases of this transient, one representing the population of the M*.ATP state and the other representing the population of the M**.ADP.Pi state. We conclude that two motionally distinct states of the myosin head are present during ATP hydrolysis and that these states represent distinct conformational states that can be correlated with specific biochemical intermediates. Since specific labeling of myosin heads with IASL has been achieved in skinned muscle fibers, this study establishes the feasibility for the first direct detection and resolution of myosin's conformational transients during muscle contraction.

"Caged" phenylephrine: development and application to probe the mechanism of alpha-receptor-mediated vasoconstriction

A "caged" analogue of the alpha-adrenergic receptor agonist phenylephrine (PE) was prepared by exploiting the 2-nitrobenzyl protecting group and using a synthetic procedure developed to permit preferential derivatization at the amino group. On isolated adult rat mesenteric arterioles, caged-PE had no measurable effects at concentrations up to 100 microM; 0.5-ms light flashes in the presence of caged-PE, however, produced marked and dose-dependent vasoconstriction. Flash-induced vasoconstrictions were blocked by the alpha-receptor antagonist phentolamine and were unaffected by the beta-receptor antagonist propranolol, indicating that the light-induced responses reflect the selective activation of alpha-adrenergic receptors. After a single flash, a large transient decrease in vessel diameter was recorded, and in most vessels, this was followed by a smaller, sustained constriction. The sustained component of the contraction was selectively eliminated when Ca2+ was removed from the bath, which suggests that different mechanisms underlie the transient and the sustained responses to PE. The responses to single flashes of varying intensities occurred with a mean latency of 460 ms, which is consistent with the intermediacy of several steps between alpha-receptor activation and contraction. We anticipate that it will be possible to extend this approach to develop caged analogues of other neurotransmitters for mechanistic and kinetic studies.

Reaction-induced infrared difference spectroscopy for the study of protein function and reaction mechanisms

Infrared spectroscopic methods have been developed in the past decade to a sensitivity and selectivity which renders them useful for the study of enzyme function and enzyme reaction mechanisms. Originally developed as difference techniques for the investigation of light-induced reactions of photoreactive proteins, and matured in the field of bacteriorhodopsin and rhodopsin, they can now be used for the study of redox proteins by the use of electrochemical cells, or for the study of many different enzymes by the use of photolabile effector molecules. This brief review summarizes the currently available methods of infrared difference spectroscopy, the technical prerequisites, achievements and limitations.

Time-resolved electron microscopic analysis of the behavior of myosin heads on actin filaments after photolysis of caged ATP

The interaction between myosin subfragment 1 (S1) and actin filaments after the photolysis of P3-1-(2-nitrophenyl)ethyl ester of ATP (caged ATP) was analyzed with a newly developed freezing system using liquid helium. Actin and S1 (100 microM each) formed a ropelike double-helix characteristic of rigor in the presence of 5 mM caged ATP at room temperature. At 15 ms after photolysis, the ropelike double helix was partially disintegrated. The number of S1 attached to actin filaments gradually decreased up to 35 ms after photolysis, and no more changes were detected from 35 to 200 ms. After depletion of ATP, the ropelike double helix was reformed. Taking recent analyses of actomyosin kinetics into consideration, we concluded that most S1 observed on actin filaments at 35-200 ms are so called "weakly bound S1" (S1.ATP or S1.ADP.Pi) and that the weakly bound S1 under a rapid association-dissociation equilibrium with actin filaments can be captured by electron microscopy by means of our newly developed freezing system. This enabled us to directly compare the conformation of weakly and strongly bound S1. Within the resolution of deep-etch replica technique, there were no significant conformational differences between weakly and strongly bound S1, and neither types of S1 showed any positive cooperativity in their binding to actin filaments. Close comparison revealed that the weakly and strongly bound S1 have different angles of attachment to actin filaments. As compared to strongly bound S1, weakly bound S1 showed a significantly broader distribution of attachment angles. These results are discussed with special reference to the molecular mechanism of acto-myosin interaction in the presence of ATP.

Postsynaptic activation at the squid giant synapse by photolytic release of L-glutamate from a 'caged' L-glutamate

1. Pharmacological evidence suggests L-glutamate is a strong candidate as a transmitter at the giant synapse of the squid. Postsynaptic activation at the giant synapse cannot be effected by conventional application of putative neurotransmitters by iontophoresis or perfusion, apparently because the complex structure of the synapse prevents a sufficiently rapid change in concentration at the postsynaptic membrane. Flash photolytic release of L-glutamate from a pharmacologically inert 'caged' L-glutamate pre-equilibrated in the stellate ganglion of Alloteuthis or Loligo was used to determine whether L-glutamate can produce postsynaptic activation when released rapidly in the synaptic clefts. 2. The preparation, reaction mechanism and properties of the caged L-glutamate, N-1-(2-nitrophenyl)ethoxycarbonyl-L-glutamate, are described. The product quantum yield on photolysis was 0.65 (+/- 0.05). On flash photolysis glutamate release followed a single exponential time-course in the pH range 5.5-7.8. The rate constant was proportional to [H+] and was 93 s-1 at pH 5.5 and 16 degrees C in artificial sea water (ionic strength, I = 0.68 M). 3. At pH 7.8 flash photolysis of caged glutamate pre-equilibrated in the synapse caused only a slow depolarization. A second photolytic release of L-glutamate or transsynaptic activation produced no further depolarization, suggesting desensitization and inactivation of postsynaptic mechanisms by the initial pulse of L-glutamate. 4. Synaptic transmission in the giant synapse was normal at pH 5.5. Flash photolysis at pH 5.5 caused rapid production of L-glutamate within the synaptic cleft and a fast postsynaptic depolarization which generated postsynaptic action potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid release of an alpha-adrenergic receptor ligand from photolabile analogues

A series of 2-nitrobenzyl derivatives of the alpha 1-selective adrenergic agonist, L-phenylephrine [(R)-N-[2-(3-hydroxyphenyl)-2-hydroxyethyl]-N-methylammonium chloride], have been synthesized and characterized for the purpose of developing biologically inert compounds that can be rapidly converted to L-phenylephrine by near-UV irradiation. The compounds, derivatized on the phenolic oxygen, were O-(1-(2-nitrophenyl)ethyl)phenylephrine (I), O-(2-nitrobenzyl)phenylephrine (II), O-(4,5-dimethoxy-2-nitrobenzyl)phenylephrine (III), and O-(alpha-carboxyl-2-nitrobenzyl)phenylephrine (IV). All four compounds photolyzed to free phenylephrine following a brief exposure to 300-350-nm light or 347-nm laser light with steady-state quantum yields ranging from 0.05 to 0.28. The rates of phenylephrine formation on photolysis were estimated from the decay rates of aci-nitro intermediates detected by absorbance between 380 and 500 nm. Compound IV displayed the highest quantum yield (0.28) and most rapid photolysis rate (1980 s-1) measured under near physiological conditions, pH 7.0, 22 degrees C. Biological properties of the compounds were examined in smooth muscle from rat caudal artery. Laser pulse photolysis of IV at 347 nm initiated a maximal contraction in Krebs buffer, pH 7.1, 25 degrees C, that mimicked the response to 50 microM phenylephrine but was faster in onset. Photoinitiated contractions were characterized by a delay of 0.93 +/- 0.09 s followed by a rising phase with a 10-90% rise time of 3.56 +/- 0.17 s (n = 7). Responses were fully blocked by the alpha 1-selective antagonist prazosin.(ABSTRACT TRUNCATED AT 250 WORDS)

Confocal microfluorimetry of Ca2+ signals evoked in Xenopus oocytes by photoreleased inositol trisphosphate

1. The subcellular characteristics of inositol 1,4,5-trisphosphate (InsP3)-induced Ca2+ liberation were studied in Xenopus oocytes by the use of confocal microfluorimetry to monitor Ca2+ signals from minutely localized region of the cell in response to photorelease of InsP3 from a caged precursor. 2. Photorelease of increasing amounts of InsP3 by progressively longer light flashes evoked transient Ca2+ responses that appeared abruptly at a certain threshold duration, and then grew steeply over a narrow range of flash durations to reach a maximum. Further lengthening of flash duration gave no increase in size of the Ca2+ signals, but their rate of rise continued to increase and their duration became longer. Simultaneous measurements of Ca(2+)-activated Cl- currents showed a slightly higher threshold than the Ca2+ signal, and a more graded dependence upon flash duration. 3. The threshold flash durations required to evoke Ca2+ and membrane current signals grew by more than 100-fold as the area of the oocyte exposed to photolysis light was reduced from a square of 140 microns to 5 microns. 4. Ca2+ signals evoked by photoreleased InsP3 began following a dose-dependent latency that was as long as several seconds with low intensity light, but shortened to about 50 ms at maximum intensity. The extrapolated minimum latency with infinite photorelease of InsP3 was about 30 ms. 5. InsP3-evoked membrane currents began 30 ms or longer after the corresponding Ca2+ signals, whereas currents evoked by photorelease of Ca2+ from a caged precursor began within 5 ms of the onset of the light flash. 6. No differences in duration of InsP3-evoked Ca2+ signals were apparent when the confocal measuring spot was positioned close to the plasma membrane or about 10 microns more deeply into the oocyte. At both locations the Ca2+ signals were more prolonged than the associated membrane current signals. 7. Ca2+ signals to a test light flash were suppressed for about 2 s following a conditioning suprathreshold flash, but recovered almost completely after 6 s. The associated membrane current signals were facilitated at short intervals, suppressed at intervals between 0.5 and 3 s, and subsequently recovered more slowly than the Ca2+ signals. 8. Photorelease of InsP3 during 30 s exposures of low intensity evoked trains of repetitive Ca2+ spikes. The overall amplitudes of these responses changed little with increasing in frequency, and became smaller and superimposed on a more sustained elevation of Ca2+.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium ions are involved in Escherichia coli chemotaxis

Escherichia coli regulates intracellular free Ca2+ at about 90 nM [Gangola, P. & Rosen, B. P. (1987) J. Biol. Chem. 262, 12570-12574]. To increase intracellular free Ca2+, nitr-5/Ca2+, a "caged" Ca2+ compound, was electroporated into cells and then its affinity for Ca2+ was reduced by exposure to 370-nm light. Upon release of the Ca2+ ions, the cells tumbled. Studies on mutant strains showed that the receptor proteins (methyl-accepting chemotaxis proteins, MCPs) were not required for the Ca(2+)-induced tumbling but that CheA, CheW, and CheY proteins were required. Similar results were obtained with DM-nitrophen/Ca2+, another caged calcium compound that releases Ca2+ upon illumination at 340 nm. Diazo-2, a caged Ca2+ chelator that takes up Ca2+ upon illumination at 340 nm, was used to decrease intracellular free Ca2+, and this caused smooth swimming.

Potentiation of inositol trisphosphate-induced Ca2+ mobilization in Xenopus oocytes by cytosolic Ca2+

1. The ability of cytosolic Ca2+ ions to modulate inositol 1,4,5-trisphosphate (Insp3)-induced Ca2+ liberation from intracellular stores was studied in Xenopus oocytes using light flash photolysis of caged InsP3. Changes in cytosolic free Ca2+ level were effected by inducing Ca2+ entry through ionophore and voltage-gated plasma membrane channels and by injection of Ca2+ through a micropipette. Their effects on Ca2+ liberation were monitored by video imaging of Fluo-3 fluorescence and by voltage clamp recording of Ca(2+)-activated membrane Cl- currents. 2. Treatment of oocytes with the Ca2+ ionophores A23187 and ionomycin caused a transient elevation of cytosolic Ca2+ level when cells were bathed in Ca(2+)-free solution, which probably arose because of release of Ca2+ from intracellular stores. 3. Membrane current and Fluo-3 Ca2+ signals evoked by photoreleased InsP3 in ionophore-treated oocytes were potentiated when the intracellular Ca2+ level was elevated by raising the Ca2+ level in the bathing solution. 4. Responses to photoreleased InsP3 were similarly potentiated following activation of Ca2+ entry through voltage-gated Ca2+ channels expressed in the plasma membrane. 5. Ca(2+)-activated membrane currents evoked by depolarization developed a delayed 'hump' component during sustained photorelease of InsP3, probably because Ca2+ ions entering through the membrane channels triggered liberation of Ca2+ from intracellular stores. 6. Ba2+ and Sr2+ ions were able to substitute for Ca2+ in potentiating InsP3-mediated Ca2+ liberation. 7. Gradual photorelease of InsP3 by weak photolysis light evoked Ca2+ liberation that began at particular foci and then propagated throughout, but not beyond that area of the oocyte exposed to the light. Local elevations of intracellular Ca2+ produced by microinjection of Ca2+ acted as new foci for the initiation of Ca2+ liberation by InsP3. 8. In resting oocytes, intracellular injections of Ca2+ resulted only in localized elevation of intracellular Ca2+, and did not evoke propagating waves. 9. The results show that cytosolic Ca2+ ions potentiate the ability of InsP3 to liberate Ca2+ from intracellular stores. This process may be important for the positive feedback mechanism underlying the generation of Ca2+ spikes and waves, and for interactions between the InsP3 pathway and Ca2+ ions entering cells through voltage- and ligand-gated channels.

Calcium-dependent immediate feedback control of inositol 1,4,5-triphosphate-induced Ca2+ release

The temporal and spatial distribution of increases in intracellular Ca2+ concentration is an important factor in cellular signal transduction. Inositol 1,4,5-trisphosphate (InsP3) plays a key part in agonist-induced Ca2+ release, which can take place abruptly and in a confined space by a mechanism that is not fully understood. Here we analyse the kinetics of InsP3-induced Ca2+ release following flash photolysis of caged InsP3 or caged Ca2+, and demonstrate that Ca(2+)-dependent immediate feedback control is an important determinant of the time course of Ca2+ release. The positive feedback mechanism is also important for the 'loading dependence' of InsP3-induced Ca2+ release. Furthermore, our results support the operation of positive cooperativity in channel opening and feedback control augments the steep InsP3 concentration-Ca2+ release relation. These inherent properties of InsP3-induced Ca2+ release are expected to give rise to temporally abrupt and/or spatially confined Ca2+ release within the cell.

Time-resolved protein crystallography

Advances in synchrotron radiation technology have allowed exposure times from protein crystals of the order of milliseconds to be used routinely, and in exceptional circumstances exposure times of 100 ps have been obtained. However, many data sets take seconds to record because of the slow time scale of film change or crystal reorientation or translation when more than one exposure is required. This problem has been addressed by Amemiya et al. (1989). There has been considerable progress in methods to initiate reactions in protein crystals, especially the development of photolabile caged compounds but also temperature jump, pH jump, and diffusion. Although flash lamps deliver pulses of 100 mJ/ms, often several pulses are required to release sufficient product, and reaction initiation can take several seconds. Laser illumination can provide more powerful input, but the laser must be accommodated within the restricted space at the synchrotron station. The requirement to maintain synchrony among the molecules in the crystal lattice as the reaction proceeds and to ensure that the lifetime of intermediates is longer than data collection rates emphasizes the need for chemical characterization of the reaction under study. As Ringe advocated in the studies with chymotrypsin, it may be more profitable to devise conditions under which certain intermediates along the reaction pathway accumulate in the crystal and to record these in a series of discrete steps rather than continuous monitoring of the reaction. The Laue method is limited to those proteins that give well-ordered crystals and problems of transient disorder on initiation of reaction and problems of radiation damage need to be overcome or avoided by suitable experimental protocols.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of caged substrates to assess the activity of 6-phosphogluconate dehydrogenase in living sea urchin eggs

As part of our inquiries into the regulation of the hexose monophosphate shunt in the early development of sea urchin eggs and embryos, we have developed a novel method to assess the in vivo activity of the enzyme 6-phosphogluconate dehydrogenase (6PGDH) before and after fertilization. Our measurements show that the intracellular level of 6-phosphogluconate (6PG) in eggs decreases 60% after fertilization, which is consistent with the increase in the activity of 6PGDH previously reported using irreversibly permeabilized cell assays (Swezey and Epel, Proc. Natl. Acad. Sci USA 85, 812-816, 1988). The in vivo turnover of the 6PG pool was assessed using a new radioisotopic technique. 1-14C-labeled 6PG was chemically modified such that it was not metabolized by cellular 6PGDH and could be rapidly converted back to 6PG by photolysis. This "caged" 6PG was introduced into unfertilized sea urchin eggs using a transient permeabilization procedure, and then the oxidation of [1-14C]6PG in vivo upon irradiation was followed. Oxidation of 6PG was complete within 7-11 s of irradiation, indicating an extremely rapid turnover of this pool in sea urchin eggs. Based on the 6PG pool sizes and the kinetic properties of 6PGDH, determined here, along with the activity levels seen in permeabilized cells, the half-time for the label in the 6PG pool in sea urchin eggs is calculated to be 26 s. This is inconsistent with the in vivo turnover rates seen in these studies, indicating that the permeabilized cell assays overestimate the degree of inhibition of 6PGDH before fertilization. These results suggest that caution should be exercised in extrapolating data obtained from permeabilized cells to the situation in vivo.

Changes in the lateral diffusion of fluorescent lipid analogues in the surface membrane of adult male Schistosoma mansoni

The effect of serotonin on the fluidity of the tegumental membranes of adult male Schistosoma mansoni was assessed by the fluorescence recovery after photobleaching technique. It was demonstrated that the translational diffusion of 5-N'-octadecanoyl aminofluorescein is reduced by a mechanism involving G-protein coupled activation of adenylate cyclase and lowering of intracellular calcium concentration. Furthermore, the lateral diffusion coefficient and the mobile fraction appear to be controlled by calcium and cAMP dependent pathways respectively. No change in the diffusion of the fluorescent phospholipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-phosphatidyl choline was observed, suggesting the two probes used here partition into two different domains that are under independent control. An increase in the amount of protein associating with a membrane cytoskeleton is also demonstrated.

How fast does an acetylcholine receptor channel open? Laser-pulse photolysis of an inactive precursor of carbamoylcholine in the microsecond time region with BC3H1 cells

The integrated function of the nervous system depends on specific and rapid transmission of signals between its constituent cells. The nicotinic acetylcholine receptor is the best known of a group of membrane-bound proteins responsible for such transmission; for this process to occur, a specific neurotransmitter, in this case acetylcholine, must bind to the receptor, which then forms transmembrane channels through which cations pass. The resulting change in transmembrane voltage determines whether or not a signal is transmitted. The question of how fast this process takes place in any neurotransmitter receptor has remained one of the interesting and most challenging in the field. To answer it, many attempts have been made to evaluate the rate constant for the opening of the acetylcholine receptor channel, but in almost all these studies the rate was measured after the receptor-mediated reaction, which involves the open channel and many intermediate states, had reached a quasi equilibrium. This resulted in a plethora of reported values for the rate constant that differ by a factor of up to 50-fold, even when the measurements were made with the same type of cell. The new approach described here involves the use of single cells of a mammalian cell line (BC3H1), containing muscle-type acetylcholine receptors, and the rapid introduction of neurotransmitter to the cell surface. The rapid delivery was achieved by converting a previously synthesized photolabile precursor of carbamoylcholine to carbamoylcholine, a stable amino-group-containing analogue of acetylcholine, with a single laser pulse and an observed photolysis rate of 7300 s-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Bacterial chemoreceptor signaling probed by flash photorelease of a caged serine

A caged serine, a photolabile compound that liberates serine upon photolysis, has been synthesized. Smooth-swimming responses of the bacterium Escherichia coli to caged serine photorelease were videotaped. The mean latency was measured from the videorecords using computerized motion analysis. This time was approximately 0.2 s. Caged photorelease of a photolabile but nonchemotactic serine analogue had no effect on the swimming behavior of the bacteria. A tumbly mutant strain lacking tsr, the serine chemoreceptor, did not respond to caged serine photorelease.

Effects of alcohols on responses evoked by inositol trisphosphate in Xenopus oocytes

1. The effects of ethanol and other alcohols on inositol 1,4,5-trisphosphate (InsP3) signalling were studied in Xenopus oocytes by the use of flash photolysis of caged InsP3. Calcium liberation induced by InsP3 was monitored by voltage-clamp recording of Ca(2+)-activated membrane currents, and by fluorescence of the Ca2+ indicator Fluo-3. 2. Membrane current and fluorescence Ca2+ signals evoked by light flashes giving small responses were initially potentiated by bath application of ethanol (80-400 mM). However, the responses subsequently declined while ethanol was present and were strongly reduced or suppressed when it was removed. 3. These effects did not arise artifactually from changes in photolysis of caged InsP3, as similar results were seen with responses evoked by intracellular injections of InsP3. Also, the effects on the membrane current did not arise primarily through actions on the Ca(2+)-dependent Cl- channels, since currents evoked by intracellular injections of Ca2+ were little changed by ethanol. 4. Ethanol reduced the threshold level of InsP3 required to cause Ca2+ liberation. Thus, potentiation was most prominent with small responses evoked by brief light flashes, whereas the predominant effect on larger responses was inhibitory. 5. The facilitatory and inhibitory actions of ethanol persisted after removing extracellular Ca2+. 6. Intracellular injections of ethanol produced an initial inhibition of InsP3 responses, followed, in some oocytes, by a potentiation. 7. Methanol had little effect on InsP3 responses, whereas butanol and other long-chain alcohols produced strong inhibition, but little or no potentiation. 8. We conclude that extracellular application of ethanol produces a rapid potentiation of InsP3-mediated Ca2+ liberation, and a more slowly developing inhibition. The potentiation may arise through stimulation of InsP3 formation at the plasma membrane, whereas the inhibition occurs more deeply in the cell. Both actions were evident at relatively low concentrations (a few tens of millimoles per litre), and might thus be important in the behavioural effects of ethanol intoxication.

Control of pro-oxidant activity of cupric ions by entrapment in unilamellar lipid vesicles

As a demonstration of a potential means of delivering and controlling the biochemical and biological activity of metal ions, cupric ions have been trapped in unilamellar phospholipid vesicles. The activity of these cupric ion-containing vesicles as catalysts of the autoxidation of ascorbate and epinephrine has been investigated. A marked increase in autoxidation rate was observed on release of the cupric ion on addition of detergent. When an azobenzene-containing photochromic lipid was incorporated in the bilayer membrane of the vesicles, the release of cupric ions could be initiated by irradiation with ultraviolet light. In the dark, these vesicles remained stable for at least several weeks. Photo-controlled release of liposomally-entrapped species might find application in areas similar to those where 'caged' reagents are presently used.

Identification of signaling states of a sensory receptor by modulation of lifetimes of stimulus-induced conformations: the case of sensory rhodopsin II

Lifetimes of stimulus-induced conformations of the phototaxis receptor sensory rhodopsin II (SR-II) from Halobacterium halobium are modulated with seven receptor analogues. By monitoring the receptor dynamics in vitro and physiological responses of the cell in vivo, we observe receptor signaling efficiency increases with decreasing cycling frequency (turnover number) of the receptor. The results demonstrate that modulating lifetimes of protein conformations at the SR-II photoactivation site with chromophore analogues alters the lifetime of the active conformation at the signaling site. We further explore the relationship between photocycle intermediates and the signaling efficiency by analyzing the time-averaged concentrations of the two long-lived spectral intermediates of the SR-II photocycle: S-II350 and S-II530. The results are consistent with the signaling site being activated during formation of S-II350, but not reset by the transition of S-II350 into S-II530; rather deactivation appears to require subsequent decay of S-II530. The results indicate the structural changes at the photoactivation site in the S-II350----S-II530 transition do not reset the signaling site. The procedure used here, applicable in principle to any photoactivated or ligand-activated receptor, provides an initial approach to identify structural alterations key to the receptor activation process.

Time-resolved cryo-electron microscopic study of the dissociation of actomyosin induced by photolysis of photolabile nucleotides

The rapid release of a substrate or other ligand from photolabile precursors in a thin layer suspension of biological specimens followed by rapid freezing provides a method of trapping and visualizing short-lived states in a dynamic system. We demonstrate here the first successful application of this method to study the interaction of actin filaments with myosin subfragment 1 (S1) after release of nucleotides. The results obtained suggest that structural changes in actin filaments occur as a result of interaction with S1.

Restoring forces in cardiac myocytes. Insight from relaxations induced by photolysis of caged ATP

Concentration jumps of intracellular ATP were produced by photolysis of P3-1-(2-nitrophenyl)ethyl (NPE)-caged ATP and were used to investigate the passive relengthening properties in unloaded cardiac myocytes. Patch-clamp pipettes in the whole-cell mode were used to voltage-clamp the myocytes and to load the cells with caged ATP while optical methods were applied to record sarcomere length or cell length simultaneously. Cell length was varied using energy deprivation contractures while intracellular Ca2+ was controlled with EGTA. At sarcomere lengths between 1.8 and 1.4 microns cellular relengthening after photolysis of caged ATP was rapid (t1/2 approximately 100 ms) and could be well described by a simple mechanical model. However, ATP jumps made at sarcomere lengths approximately 1.1 microns led to slow relengthening (t1/2 approximately seconds), comparable to the slow reextensions observed in skinned myocytes after bulk solution changes. We attribute the slow and incomplete relengthening of intact and skinned myocytes after severe rigor shortening to deformation and alteration of structural elements inside the cell. Relengthening from intermediate sarcomere lengths in intact cells is elastic and provides information about the underlying relengthening forces inside the cell. The data do not support the presence of a significant discontinuity in elastic modulus at a sarcomere length of approximately 1.6 microns expected from ultrastructural features of the sarcomeres and from observations in skinned myocytes. Our results suggest that the cell length measurements usually performed in this preparation provide an adequate description of the force produced by the unloaded cell in the steady state. The results also provide a way to estimate the error arising from viscous forces during rapid shortening.

Magnetic resonance of membranes

Images

Caffeine inhibits inositol trisphosphate-mediated liberation of intracellular calcium in Xenopus oocytes

1. Voltage-clamp recording of Ca(2+)-activated chloride currents in Xenopus oocytes was used to study the effects of caffeine on the liberation of intracellular Ca2+ induced by photo-release of inositol 1,4,5-trisphosphate (InsP3) from caged InsP3. Bath application of caffeine, at concentrations between 0.1 and 10 mM, reduced or abolished the current evoked by photo-release of InsP3 and by microinjection of InsP3. 2. Caffeine did not appreciably reduce currents evoked by injection of Ca2+ into oocytes, whereas measurements using the Ca2+ indicator Rhod-2 showed that it instead inhibited the liberation of Ca2+ by InsP3. 3. Caffeine increased the threshold amount of InsP3 required to evoke a current response and proportionally reduced the currents evoked by suprathreshold levels of InsP3. 4. Theophylline and 3-isobutyl-1-methylxanthine (IBMX) were much less potent than caffeine, and few changes were seen in the InsP3 responses following application of forskolin or intracellular injection of cyclic AMP. Thus, inhibition of InsP3 responses by caffeine does not arise through inhibition of phosphodiesterase enzymes. 5. Even at high (10 mM) concentrations, caffeine did not itself elicit any clear Ca(2+)-activated current. It is therefore unlikely that inhibition of the InsP3 responses arise because caffeine itself liberates Ca2+ from intracellular stores. 6. The site of action of caffeine is intracellular, because injections of caffeine into the oocyte strongly inhibited responses to InsP3, whereas local extracellular applications of similar amounts were almost without effect.

Ca2+ and activation mechanisms in skeletal muscle

It has been known for a number of years that calcium ions play a crucial role in excitation-contraction (e-c) coupling (Sandow, 1952). The majority of the calcium required for this process is derived, at least in vertebrate striated muscle fibres, from discrete intracellular stores located at sites within the cell: the terminal cysternae (tc)/junctional SR of the sarcoplasmic reticulum (SR) (Fig. 1 a). These storage sites not only form a compartment that is distinct from the sarcoplasm of the fibre, but they are also closely associated with the contractile elements, the myofibrils. The SR release sites are activated following the spread of electrical activity (Huxley and Taylor, 1958) along the transverse (T) tubular system (Eisenberg and Gage, 1967; Adrian et al. 1969a, b; Peachey, 1973) from the surface membrane (Bm).

Localized all-or-none calcium liberation by inositol trisphosphate

Laser confocal microscopy was used to monitor calcium ion (Ca2+) liberation from highly localized (micrometer) regions of intact Xenopus oocytes in response to photo-released inositol 1,4,5-trisphosphate (InsP3). Local Ca2+ release varied in an all-or-none manner with increasing amount of InsP3, in contrast to signals recorded from larger areas, which grew progressively as the concentration of InsP3 was raised above a threshold. Liberation of Ca2+ was restricted to within a few microns of the site of InsP3 release and, in response to agonist activation, localized regions of the oocyte showed asynchronous oscillations in cytoplasmic Ca2+ release. Results obtained with this technique provided direct evidence that InsP3-induced Ca2+ liberation was quantized and suggest that the InsP3-sensitive Ca2+ pool may be a collection of independent, localized compartments that release Ca2+ in an all-or-none manner.

Myosin heads have a broad orientational distribution during isometric muscle contraction: time-resolved EPR studies using caged ATP

To study the orientation of spin-labeled myosin heads in the first few seconds after the production of saturating ATP, we have used a laser flash to photolyze caged ATP during EPR data acquisition. Rabbit psoas muscle fibers were labeled with maleimide spin label, modifying 60% of the myosin heads without impairing muscle fiber biochemical and physiological activity (ATPase and force). The muscle bundles were incubated for 30 min with 5 mM caged ATP prior to the UV flash. The flash, from an excimer laser, liberated 2-3 mM ATP, generating maximum force in the presence of Ca2+ and relaxing fully in the absence of Ca2+. Control experiments, using fibers decorated with labeled myosin subfragment, showed that the flash liberates sufficient ATP to saturate myosin active sites in all regions of the muscle bundles. To increase the time resolution, and to minimize the time of the contraction, we followed in time the intensity at a single spectral position (P2), which is associated with the high degree of orientational order in rigor. ATP liberation produced a rapid decrease of P2 with liberation of ATP, indicating a large decrease in orientational order in both relaxation and contraction. This transient was absent when caged AMP was used, ruling out nonspecific effects of the UV flash and subsequent photochemistry. The steady-state level of P2 during contraction was almost as low as that reached in relaxation, although the duration of the steady state was much more brief in contraction. Upon depletion of ATP in contraction, the P2 intensity reverted to the original rigor level, accompanied by development of rigor tension. The steady-state results obtained in the brief contractions induced by caged ATP are quantitatively consistent with those obtained in longer contractions by continuously perfusing fibers with ATP. In isometric contraction, most (88% +/- 4%) of the heads are in a population characterized by a high degree of axial disorder, comparable to that observed for all heads in relaxation. Since the stiffness of these fibers in contraction is 80% of the stiffness in rigor, it is likely that most of the heads in this highly disoriented population are attached to actin in contraction and that most actin-attached heads in contraction are in this disoriented population.

Two-photon laser scanning fluorescence microscopy

Molecular excitation by the simultaneous absorption of two photons provides intrinsic three-dimensional resolution in laser scanning fluorescence microscopy. The excitation of fluorophores having single-photon absorption in the ultraviolet with a stream of strongly focused subpicosecond pulses of red laser light has made possible fluorescence images of living cells and other microscopic objects. The fluorescence emission increased quadratically with the excitation intensity so that fluorescence and photo-bleaching were confined to the vicinity of the focal plane as expected for cooperative two-photon excitation. This technique also provides unprecedented capabilities for three-dimensional, spatially resolved photochemistry, particularly photolytic release of caged effector molecules.

Inhibition by Ca2+ of inositol trisphosphate-mediated Ca2+ liberation: a possible mechanism for oscillatory release of Ca2+

Light-flash photolysis of caged inositol 1,4,5-trisphosphate (InsP3) was used to generate reproducible transients of free InsP3 in Xenopus oocytes, and the resulting liberation of Ca2+ from intracellular stores was monitored by recording Ca2+-activated membrane currents and by use of the fluorescent Ca2+ indicator fluo-3. InsP3-mediated Ca2+ release was inhibited by elevating the intracellular free Ca2+ level, either by microinjecting Ca2+ into the cell or by applying conditioning light flashes to liberate Ca2+. This inhibition followed a slow time course, being maximal after about 2 s and subsequently declining over several seconds. Negative feedback of Ca2+ ions on InsP3-mediated Ca2+ liberation may explain the oscillatory release of Ca2+ seen during activation of inositol phospholipid signaling in the oocyte, and the time course of the inhibition is consistent with the period of the oscillations.