Flash lamp-based irradiation of caged compounds

Technical approaches and challenges to study AMPA receptors in oligodendrocyte lineage cells: Past, present, and future

Receptors for α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPARs) are ligand-gated ionotropic receptors for glutamate that is a major excitatory neurotransmitter in the central nervous system. AMPARs are located at postsynaptic sites of neuronal synapses where they mediate fast synaptic signaling and synaptic plasticity. Remarkably, AMPARs are also expressed by glial cells. Their expression by the oligodendrocyte (OL) lineage cells is of special interest because AMPARs mediate fast synaptic communication between neurons and oligodendrocyte progenitor cells (OPCs), modulate proliferation and differentiation of OPCs, and may also be involved in regulation of myelination. On the other hand, during pathological conditions, AMPARs may mediate damage of the OL lineage cells. In the present review, we focus on the technical approaches that have been used to study AMPARs in the OL lineage cells, and discuss future perspectives of AMPAR research in these glial cells.

Caged neurotransmitters and other caged compounds: design and application

The approaches using caged neurotransmitters described here enable transient kinetic investigations to be made with membrane-bound proteins (receptors) on a cell surface with the same time resolution as was previously possible only with proteins in solution.

Flash photolysis of caged compounds in the cilia of olfactory sensory neurons

Photolysis of caged compounds allows the production of rapid and localized increases in the concentration of various physiologically active compounds. Caged compounds are molecules made physiologically inactive by a chemical cage that can be broken by a flash of ultraviolet light. Here, we show how to obtain patch-clamp recordings combined with photolysis of caged compounds for the study of olfactory transduction in dissociated mouse olfactory sensory neurons. The process of olfactory transduction (Figure 1) takes place in the cilia of olfactory sensory neurons, where odorant binding to receptors leads to the increase of cAMP that opens cyclic nucleotide-gated (CNG) channels. Ca entry through CNG channels activates Ca-activated Cl channels. We show how to dissociate neurons from the mouse olfactory epithelium and how to activate CNG channels or Ca-activated Cl channels by photolysis of caged cAMP or caged Ca. We use a flash lamp to apply ultraviolet flashes to the ciliary region to uncage cAMP or Ca while patch-clamp recordings are taken to measure the current in the whole-cell voltage-clamp configuration.

Photo-activatable probes for the analysis of receptor function in living cells

Photo-activatable (caged) probes are powerful research tools for biological investigation. The superb maneuverability of a light beam allows researchers to activate caged probes with pinpoint accuracy. Recent developments in caging chemistry and two-photon excitation technique further enhance our capability to perform photo-uncaging with even higher spatial and temporal resolution, offering new photonic approaches to study cell signaling dynamics in greater detail. Here we present a sample method that combines the techniques of photo-activation and digital fluorescence microscopy to assay an important class of intracellular receptors for the second messenger D-myo-inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3), or IP(3)). The imaging assay is performed in fully intact living cells using a caged and cell membrane permeable ester derivative of IP(3), cm-IP(3)/PM.

Implementation of a flash-photolysis system for time-resolved cryo-electron microscopy

We describe here the implementation of a flash-photolysis system for time-resolved cryo-electron microscopy. A previously designed computer-controlled cryo-plunging apparatus [White, H.D., Thirumurugan, K., Walker, M.L., Trinick, J., 2003. A second generation apparatus for time-resolved electron cryo-microscopy using stepper motors and electrospray. J. Struct. Biol. 144, 246-252] was used as a hardware platform, onto which a xenon flash lamp and liquid light pipe were mounted. The irradiation initiates a reaction through cleavage of the photolabile blocking group from a biologically active compound. The timespan between flashing and freezing in cryogen is on the order of milliseconds, and defines the fastest observable reaction. Blotting of excess fluid, which takes on the order of 1s, is done before irradiation and thus does not represent a rate-limiting step. A specimen-heating problem, identified by measurements with a thermocouple, was alleviated with the use of thick, aluminum-coated grids.

Lung surfactant secretion by interalveolar Ca2+ signaling

Although clusters of alveoli form the acinus, which is the most distal respiratory unit, it is not known whether interalveolar communication coordinates acinar surfactant secretion. To address this, we applied real-time digital imaging in conjunction with photo-excited Ca2+ uncaging in intact alveoli of the isolated, blood-perfused rat lung. We loaded alveolar cells with the Ca2+ cage o-nitrophenyl EGTA-AM (NP-EGTA-AM) together with the fluorophores, fluo 4, or LysoTracker green (LTG) to determine, respectively, the cytosolic Ca2+ concentration ([Ca2+]cyt) or type 2 cell secretion. To uncage Ca2+ from NP-EGTA, we exposed a region in a selected alveolus to high-intensity UV illumination. As a result, fluo 4 fluorescence increased, whereas LTG fluorescence decreased, in the photo-targeted region, indicating that uncaging both increased [Ca2+]cyt and induced secretion. Concomitantly, [Ca2+]cyt increases conducted from the uncaging site induced type 2 cell secretion in both the selected alveolus as well as in neighboring alveoli, indicating the presence of interalveolar communication. These conducted responses were inhibited by pretreating alveoli with the connexin43 (Cx43)-inhibiting peptides gap 26 and gap 27. However, although the conducted [Ca2+]cyt increase diminished with distance from the uncaging site, type 2 cell secretion rates were similar at all locations. We conclude that Cx43-dependent, interalveolar Ca2+ signals regulate type 2 cell secretion in adjacent alveoli. Such interalveolar communication might facilitate acinar coordination of alveolar function.

Postsynaptic modulation of AMPA- and NMDA-receptor currents by Group III metabotropic glutamate receptors in rat nucleus accumbens

Whole cell patch clamp recordings from rat nucleus accumbens neurons were made in order to study the effect of metabotropic glutamate receptors and dopamine on postsynaptic glutamate receptor mediated currents. AMPA- and NMDA-R currents were evoked by flash photolysis of caged glutamate, while spike-dependent release of neurotransmitters was prevented by adding tetrodotoxin and bicuculline to the bath solution. Spontaneous potentiation of NMDA- but not AMPA-R current was observed in the early phase of stimulation, followed by depotentiation and subsequent stabilization. The Group III metabotropic glutamate receptor antagonist MAP4 induced a transient potentiation of both AMPA- and NMDA-R current amplitudes, without affecting rise times and decay time constants. In contrast, the Group I-II metabotropic glutamate receptor antagonist MCPG and the neurotransmitter dopamine did not exert significant effects on either AMPA- or NMDA-R currents. These data suggest that at least one of the Group III subtypes is located postsynaptically in the nucleus accumbens and is able to dampen the activity of ionotropic glutamatergic receptors. In contrast, our results do not support a modulation of postsynaptic AMPA- and NMDA-R currents by Group I/II metabotropic glutamate receptors or dopamine. Modulation of both AMPA- and NMDA-R currents in the nucleus accumbens is likely to play a major role in setting the cellular excitability in response to behaviourally relevant limbic inputs, and in regulating the plasticity of these responses.

Photochemical gating of heterologous ion channels: remote control over genetically designated populations of neurons

Heterologous proteins capable of transducing physical or chemical stimuli into electrical signals can be used to control the function of excitable cells in intact tissues or organisms. Restricted genetically to circumscribed populations of cellular targets, these selectively addressable sources of depolarizing current can supply distributed inputs to neural circuits, stimulate secretion, or regulate force and motility. In an initial demonstration of this principle, we have used elements of a G protein coupled signaling system, the phototransduction cascade of the fruit fly, to sensitize generalist vertebrate neurons to light [Zemelman, B. V., Lee, G. A., Ng, M. & Miesenböck, G. (2002) Neuron 33, 15-22]. We now describe the use of ectopically expressed ligand-gated ion channels as transducers of optical or pharmacological stimuli. When either the capsaicin receptor, TRPV1, the menthol receptor, TRPM8, or the ionotropic purinergic receptor P2X(2) was introduced into hippocampal neurons, the cells responded to pulsed applications of agonist with characteristic sequences of depolarization, spiking, and repolarization. Responses required cognate matches between receptor and agonist, peaked at firing frequencies of approximately 40 Hz, initiated and terminated rapidly, and did not attenuate. Precise dose-response relationships allowed current amplitudes and firing frequencies to be tuned by varying the concentration of ligand. Agonist could be administered either pharmacologically or, in the cases of TRPV1 and P2X(2), optically, through photorelease of the active compounds from the respective "caged" precursors, 4,5-dimethoxy-2-nitrobenzyl-capsaicin and P(3)-[1-(4,5-dimethoxy-2-nitrophenyl)ethyl]-ATP.

Crystallographic structure determination of unstable species

Crystal structures of reactive short-lived species, as occurring during chemical reactions, can be determined through time-resolved crystallography or trapping approaches. Prerequisite is the initiation and characterization of the reaction in the crystal. Ways to do this, recent results, caveats, and future prospects are discussed.

Response tuning in bacterial chemotaxis

Chemotaxis of enteric bacteria in spatial gradients toward a source of chemoattractant is accomplished by increases in the length of swimming runs up the gradient. Biochemical components of the intracellular signal pathway have been identified, but mechanisms for achieving the high response sensitivity remain unknown. Binding of attractant ligand to its receptor inactivates a receptor-associated histidine kinase, CheA, which phosphorylates the signal protein CheY. The reduction in phospho-CheY, CheY-P, levels prolongs swimming runs. Here, the stimulus-response relation has been determined by measurement of excitation responses mediated by the Tar receptor to defined concentration jumps of the attractant, aspartate, administered within milliseconds by photolysis of a photolabile precursor. The bacteria responded to <1% changes in Tar occupancy when adapted to aspartate over concentrations spanning three orders of magnitude. Response amplitudes increased approximately logarithmically with stimulus strength, extending responsiveness over a greater stimulus range. The extent and form of this relation indicates that, in contrast to mechanisms for adaptive recovery, excitation signal generation involves amplification based on cooperative interactions. These interactions could entail inactivation of multiple receptor-CheA signaling complexes and/or simultaneous activation of CheY-P dephosphorylation.

Use of caged fluorochromes to track macromolecular movement in living cells

One way to visualize and track the movement of macromolecules in the living cell is to follow their movement after tagging the molecule with a 'caged' or chemically masked fluorochrome. The fluorochrome does not fluoresce until the caging group is released by spot photoactivation, and the bright fluorescent signal can then be tracked as it moves into the dark surrounding area of the cell. When coupled with rapid imaging microscopy, it is possible to measure rates of movement as fast as macromolecular diffusion. This article describes the use of photoactivatable fluorochromes to track the intracellular movement of both proteins and nucleic acids and to track cell lineages.