Development and application of caged calcium

Synthesis and Photolytic Assessment of Nitroindolinyl-Caged Calcium Ion Chelators

Neuroactive amino acids derivatised at their carboxylate groups with a photolabile nitroindolinyl group are highly effective reagents for the sub-µs release of neuroactive amino acids in physiological solutions. However, the same does not apply in the case of calcium ion chelators. In this study, nitroindolinyl-caged BAPTA is found to be completely photostable, whereas nitroindolinyl-caged EDTA photolyses only when saturated with calcium ions.

Deciphering Molecular Mechanisms and Intervening in Physiological and Pathophysiological Processes of Ca2+ Signaling Mechanisms Using Optogenetic Tools

Calcium ion channels are involved in numerous biological functions such as lymphocyte activation, muscle contraction, neurotransmission, excitation, hormone secretion, gene expression, cell migration, memory, and aging. Therefore, their dysfunction can lead to a wide range of cellular abnormalities and, subsequently, to diseases. To date various conventional techniques have provided valuable insights into the roles of Ca2+ signaling. However, their limited spatiotemporal resolution and lack of reversibility pose significant obstacles in the detailed understanding of the structure-function relationship of ion channels. These drawbacks could be partially overcome by the use of optogenetics, which allows for the remote and well-defined manipulation of Ca2+-signaling. Here, we review the various optogenetic tools that have been used to achieve precise control over different Ca2+-permeable ion channels and receptors and associated downstream signaling cascades. We highlight the achievements of optogenetics as well as the still-open questions regarding the resolution of ion channel working mechanisms. In addition, we summarize the successes of optogenetics in manipulating many Ca2+-dependent biological processes both in vitro and in vivo. In summary, optogenetics has significantly advanced our understanding of Ca2+ signaling proteins and the used tools provide an essential basis for potential future therapeutic application.

Activity-Based Sensing: A Synthetic Methods Approach for Selective Molecular Imaging and Beyond

Emerging from the origins of supramolecular chemistry and the development of selective chemical receptors that rely on lock-and-key binding, activity-based sensing (ABS)-which utilizes molecular reactivity rather than molecular recognition for analyte detection-has rapidly grown into a distinct field to investigate the production and regulation of chemical species that mediate biological signaling and stress pathways, particularly metal ions and small molecules. Chemical reactions exploit the diverse chemical reactivity of biological species to enable the development of selective and sensitive synthetic methods to decipher their contributions within complex living environments. The broad utility of this reaction-driven approach facilitates application to imaging platforms ranging from fluorescence, luminescence, photoacoustic, magnetic resonance, and positron emission tomography modalities. ABS methods are also being expanded to other fields, such as drug and materials discovery.

Optogenetic toolkit for precise control of calcium signaling

Calcium acts as a second messenger to regulate a myriad of cell functions, ranging from short-term muscle contraction and cell motility to long-term changes in gene expression and metabolism. To study the impact of Ca2+-modulated 'ON' and 'OFF' reactions in mammalian cells, pharmacological tools and 'caged' compounds are commonly used under various experimental conditions. The use of these reagents for precise control of Ca2+ signals, nonetheless, is impeded by lack of reversibility and specificity. The recently developed optogenetic tools, particularly those built upon engineered Ca2+ release-activated Ca2+ (CRAC) channels, provide exciting opportunities to remotely and non-invasively modulate Ca2+ signaling due to their superior spatiotemporal resolution and rapid reversibility. In this review, we briefly summarize the latest advances in the development of optogenetic tools (collectively termed as 'genetically encoded Ca2+ actuators', or GECAs) that are tailored for the interrogation of Ca2+ signaling, as well as their applications in remote neuromodulation and optogenetic immunomodulation. Our goal is to provide a general guide to choosing appropriate GECAs for optical control of Ca2+ signaling in cellulo, and in parallel, to stimulate further thoughts on evolving non-opsin-based optogenetics into a fully fledged technology for the study of Ca2+-dependent activities in vivo.

Light-controlled gene expression in yeast using photocaged Cu2

The manipulation of cellular function, such as the regulation of gene expression, is of great interest to many biotechnological applications and often achieved by the addition of small effector molecules. By combining effector molecules with photolabile protecting groups that mask their biological activity until they are activated by light, precise, yet minimally invasive, photocontrol is enabled. However, applications of this trendsetting technology are limited by the small number of established caged compound-based expression systems. Supported by computational chemistry, we used the versatile photolabile chelator DMNP-EDTA, long-established in neurobiology for photolytic Ca²⁺ release, to control Cu²⁺ release upon specific UV-A irradiation. This permits light-mediated control over the widely used Cu²⁺-inducible pCUP1 promoter from S. cerevisiae and thus constitutes the first example of a caged metal ion to regulate recombinant gene expression. We screened our novel DMNP-EDTA-Cu system for best induction time and expression level of eYFP with a high-throughput online monitoring system equipped with an LED array for individual illumination of every single well. Thereby, we realized a minimally invasive, easy-to-control, parallel and automated optical expression regulation via caged Cu²⁺ allowing temporal and quantitative control as a beneficial alternative to conventional induction via pipetting CuCl₂ as effector molecule.

Calcium Uncaging with Visible Light

We have designed a nitroaromatic photochemical protecting group that absorbs visible light in the violet-blue range. The chromophore is a dinitro derivative of bisstyrylthiophene (or BIST) that absorbs light very effectively (ε440 = 66,000 M(-1) cm(-1) and two-photon cross section of 350 GM at 775 nm). We developed a "caged calcium" molecule by conjugation of BIST to a Ca(2+) chelator that upon laser flash photolysis rapidly releases Ca(2+) in <0.2 ms. Using the patch-clamp method the optical probe, loaded with Ca(2+), was delivered into acutely isolated mouse cardiac myocytes, where either one- and two-photon uncaging of Ca(2+) induced highly local or cell-wide physiological Ca(2+) signaling events.

Ca2+ Uncaging in Nerve Terminals: A Three-Point Calibration Procedure

Ca(2+) uncaging can be used to create a spatially homogenous elevation of the intracellular free Ca(2+) concentration, [Ca(2+)]i, in cells. When applied to nerve terminals or secretory cells, this technique allows one to elicit transmitter release with a [Ca(2+)]i signal of measurable amplitude, and therefore to directly relate the rate of transmitter release to the measured [Ca(2+)]i. When combined with patch-clamp measurements, Ca(2+) uncaging is done by introducing a Ca(2+)-loaded photolyzable Ca(2+) chelator (like DM-nitrophen) into the cell via the whole-cell patch-pipette. A brief light pulse from a flash lamp or a pulsed laser is used to photolyze the DM-nitrophen. The resulting increase in [Ca(2+)]i is measured with ratiometric fluorescent indicators of suitable Ca(2+) affinity, such as Fura-2, Fura-4F, Fura-2FF, or Fura-6F, depending on the postflash [Ca(2+)]i values. To quantitatively measure [Ca(2+)]i, an accurate calibration of the fluorescent indicator in the presence of the photolyzable Ca(2+) chelator is necessary, which will be described here. Ca(2+) uncaging in nerve terminals has proven useful for investigating Ca(2+)-dependent functions like transmitter release, short-term plasticity, and exocytosis-endocytosis coupling in the presynaptic compartment of neurons.

An alien divalent ion reveals a major role for Ca²⁺ buffering in controlling slow transmitter release

Ca(2+)-dependent transmitter release occurs in a fast and in a slow phase, but the differential roles of Ca(2+) buffers and Ca(2+) sensors in shaping release kinetics are still controversial. Replacing extracellular Ca(2+) by Sr(2+) causes decreased fast release but enhanced slow release at many synapses. Here, we established presynaptic Sr(2+) uncaging and made quantitative Sr(2+)- and Ca(2+)-imaging experiments at the mouse calyx of Held synapse, to reveal the interplay between Ca(2+) sensors and Ca(2+) buffers in the control of fast and slow release. We show that Sr(2+) activates the fast, Synaptotagmin-2 (Syt2) sensor for vesicle fusion with sixfold lower affinity but unchanged high cooperativity. Surprisingly, Sr(2+) also activates the slow sensor that remains in Syt2 knock-out synapses with a lower efficiency, and Sr(2+) was less efficient than Ca(2+) in the limit of low concentrations in wild-type synapses. Quantitative imaging experiments show that the buffering capacity of the nerve terminal is markedly lower for Sr(2+) than for Ca(2+) (~5-fold). This, together with an enhanced Sr(2+) permeation through presynaptic Ca(2+) channels (~2-fold), admits a drastically higher spatially averaged Sr(2+) transient compared with Ca(2+). Together, despite the lower affinity of Sr(2+) at the fast and slow sensors, the massively higher amplitudes of spatially averaged Sr(2+) transients explain the enhanced late release. This also allows us to conclude that Ca(2+) buffering normally controls late release and prevents the activation of the fast release sensor by residual Ca(2+).

Measuring the rate constants of Ca²⁺ indicators

Fluorescent Ca(2+) indicators are widely used to measure the concentration of free Ca(2+) ([Ca(2+)](free)) in biological processes. To determine the exact kinetics of changes in [Ca(2+)](free) and the processes underlying these changes (e.g., Ca(2+) binding to Ca(2+)-binding proteins), it is necessary to know the exact binding properties of the indicator used. Here, we describe how to determine the binding rate constants (k(on) and k(off)) of Ca(2+) indicators.

Fluorescent measurement of [Ca²⁺]c: basic practical considerations

There is a vast array of dyes currently available for measurement of cytosolic calcium. These encompass single and dual excitation and single and dual emission probes. The choice of particular probe depends on the experimental question and the type of equipment to be used. It is therefore extremely difficult to define a universal approach that will suit all potential investigators. Preparations under investigation are loaded with the selected organic indicator dye by incubation with ester derivatives, by micropipet injection or reverse permeabilization. Indicators can also be targeted to a range of intracellular organelles. Calibration of a fluorescent signal into Ca(2+) concentration is in theory relatively simple but the investigator needs to take great care in this process. This chapter describes the theory of these processes and some of the pitfalls users should be aware of. Precise experimental details can be found in the subsequent chapters of this volume.

Photolysis of caged compounds: studying Ca(2+) signaling and activation of Ca(2+)-dependent ion channels

A wide variety of signaling molecules have been chemically modified by conjugation to a photolabile chromophore to render the substance temporarily biologically inert. Subsequent exposure to ultraviolet (UV) light can release the active moiety from the "caged" precursor in an experimentally controlled manner. This allows the concentration of active molecule to be precisely manipulated in both time and space. These techniques are particularly useful in experimental protocols designed to investigate the mechanisms underlying Ca(2+) signaling and the activation of Ca(2+)-dependent effectors.

Water-soluble, redox-active organometallic calcium chelators

This paper describes a new series of organometallic water-soluble chelators combining a redox moiety (ferrocene) and a selective Ca2+ chelator (BAPTA) separated by an ethynyl bridge. We report the synthesis and characterization of organometallic derivatives of the BAPTA chelator featuring one (2a) and two ferrocenyl (2b) moieties. Single crystal X-ray structural analysis on these chelators revealed unexpected conformations for the ferrocenyl substituent with respect to the phenyl ring of the BAPTA unit. DFT calculations on a model system of the ferrocenyl-ethynyl-BAPTA molecule were carried out to evaluate the energy separation between the two limiting conformations observed experimentally in the solid state, and to check the effective electronic communication between the binding pocket and the redox probe. The binding affinity of 2a–b for Ca2+, as probed by UV-Vis and cyclic voltammetry, revealed distinct behaviors in the presence of a metal ion depending on whether BAPTA is substituted by one or two ferrocenyl groups.

The contribution of RCK domains to human BK channel allosteric activation

Large conductance voltage- and Ca(2+)-activated K(+) (BK) channels are potent regulators of cellular processes including neuronal firing, synaptic transmission, cochlear hair cell tuning, insulin release, and smooth muscle tone. Their unique activation pathway relies on structurally distinct regulatory domains including one transmembrane voltage-sensing domain (VSD) and two intracellular high affinity Ca(2+)-sensing sites per subunit (located in the RCK1 and RCK2 domains). Four pairs of RCK1 and RCK2 domains form a Ca(2+)-sensing apparatus known as the "gating ring." The allosteric interplay between voltage- and Ca(2+)-sensing apparati is a fundamental mechanism of BK channel function. Using voltage-clamp fluorometry and UV photolysis of intracellular caged Ca(2+), we optically resolved VSD activation prompted by Ca(2+) binding to the gating ring. The sudden increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) induced a hyperpolarizing shift in the voltage dependence of both channel opening and VSD activation, reported by a fluorophore labeling position 202, located in the upper side of the S4 transmembrane segment. The neutralization of the Ca(2+) sensor located in the RCK2 domain abolished the effect of [Ca(2+)](i) increase on the VSD rearrangements. On the other hand, the mutation of RCK1 residues involved in Ca(2+) sensing did not prevent the effect of Ca(2+) release on the VSD, revealing a functionally distinct interaction between RCK1 and RCK2 and the VSD. A statistical-mechanical model quantifies the complex thermodynamics interplay between Ca(2+) association in two distinct sites, voltage sensor activation, and BK channel opening.

Measuring the kinetics of calcium binding proteins with flash photolysis

BACKGROUND

Calcium-binding proteins (CBPs) are instrumental in the control of Ca2+ signaling. They are the fastest players within the Ca2+ toolkit responding within microseconds to [Ca2+] changes. The CBPs compete for Ca2+ which plays a direct role in modulating Ca2+ transients and the resulting biochemical message. The kinetic properties of the CBPs have to be known to have a good understanding of Ca2+ signaling.

SCOPE OF REVIEW

Most techniques used to measure binding kinetics are too slow to accurately determine the fast kinetics of most CBP. Furthermore, many CBPs bind Ca2+ in a cooperative way, which should be incorporated in the kinetic modeling. Here we will review a new ultra-fast in vitro technique for measuring Ca2+ binding properties of CBPs following flash photolysis of caged Ca2+. Compartmental modeling is used to resolve the kinetics of fast cooperative Ca2+ binding to CBPs.

MAJOR CONCLUSIONS

Currently this technique has only been used to quantify the kinetics of three CBPs (calbindin, calretinin and calmodulin), but has already provided remarkable insights into the specific role that these kinetics in Ca2+ signaling.

GENERAL SIGNIFICANCE

The potential to gain novel insights into Ca2+ signaling by quantifying kinetics of other CBPs using this technique is very promising. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signaling.

Regulation of transmitter release by Ca(2+) and synaptotagmin: insights from a large CNS synapse

Transmitter release at synapses is driven by elevated intracellular Ca(2+) concentration ([Ca(2+)](i)) near the sites of vesicle fusion. [Ca(2+)](i) signals of profoundly different amplitude and kinetics drive the phasic release component during a presynaptic action potential, and asynchronous release at later times. Studies using direct control of [Ca(2+)](i) at a large glutamatergic terminal, the calyx of Held, have provided significant insight into how intracellular Ca(2+) regulates transmitter release over a wide concentration range. Synaptotagmin-2 (Syt2), the major isoform of the Syt1/2 Ca(2+) sensors at these synapses, triggers highly Ca(2+)-cooperative release above 1μM [Ca(2+)](i), but suppresses release at low [Ca(2+)](i). Thus, neurons utilize a highly sophisticated release apparatus to maximize the dynamic range of Ca(2+)-evoked versus spontaneous release.

Optical control of calcium affinity in a spiroamido-rhodamine based calcium chelator

An optically controlled Ca(2+)-chelator 1 was developed to mimic natural calcium oscillations. Compound 1, a spiroamido-rhodamine derivative of 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), underwent cycles of reversible transitions between a colorless closed state and a fluorescent open form. The closed-state exhibited a high affinity for Ca(2+) (K(d): 509 nM) with excellent selectivity over Mg(2+) (K(d): 19 mM). The open isomer had a 350-fold lower Ca(2+) affinity (K(d): 181 μM), while the Mg(2+) affinity was not significantly affected (K(d): 14 mM).

Patterning alginate hydrogels using light-directed release of caged calcium in a microfluidic device

This paper describes a simple reversible hydrogel patterning method for 3D cell culture. Alginate gel is formed in select regions of a microfluidic device through light-triggered release of caged calcium. In the pre-gelled alginate solution, calcium is chelated by DM-nitrophen (DM-n) to prevent cross-linking of alginate. After sufficient UV exposure the caged calcium is released from DM-n causing alginate to cross-link. The effect of using different concentrations of calcium and chelating agents as well as the duration of UV exposure is described. Since the cross-linking is based on calcium concentration, the cross-linked alginate can easily be dissolved by EDTA. We also demonstrate application of this capability to patterned microscale 3D co-culture using endothelial cells and osteoblastic cells in a microchannel.

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.

Synthesis, photochromic properties, and light-controlled metal complexation of a naphthopyran derivative

A light-controlled reversible binding switch based on photochromic 3H-naphtho[2,1-b]pyran is under development for studying cellular oscillatory calcium signals. The binding affinities of the closed and open forms of substituted naphthopyran 1 for Ca(2+), Mg(2+), and Sr(2+) in buffer were determined. The photochemically ring-opened form of the receptor exhibited increased affinity compared to the thermally stable closed form of the receptor. The binding affinity difference for Ca(2+) was approximately 77-fold at pH 7.6.

Caged compounds: photorelease technology for control of cellular chemistry and physiology

Caged compounds are light-sensitive probes that functionally encapsulate biomolecules in an inactive form. Irradiation liberates the trapped molecule, permitting targeted perturbation of a biological process. Uncaging technology and fluorescence microscopy are 'optically orthogonal': the former allows control, and the latter, observation of cellular function. Used in conjunction with other technologies (for example, patch clamp and/or genetics), the light beam becomes a uniquely powerful tool to stimulate a selected biological target in space or time. Here I describe important examples of widely used caged compounds, their design features and synthesis, as well as practical details of how to use them with living cells.

Ca2+ requirements for cerebellar long-term synaptic depression: role for a postsynaptic leaky integrator

Photolysis of a caged Ca(2+) compound was used to characterize the dependence of cerebellar long-term synaptic depression (LTD) on postsynaptic Ca(2+) concentration ([Ca(2+)](i)). Elevating [Ca(2+)](i) was sufficient to induce LTD without requiring any of the other signals produced by synaptic activity. A sigmoidal relationship between [Ca(2+)](i) and LTD indicated a highly cooperative triggering of LTD by Ca(2+). The duration of the rise in [Ca(2+)](i) influenced the apparent Ca(2+) affinity of LTD, and this time-dependent behavior could be described by a leaky integrator process with a time constant of 0.6 s. A computational model, based on a positive-feedback cycle that includes protein kinase C and MAP kinase, was capable of simulating these properties of Ca(2+)-triggered LTD. Disrupting this cycle experimentally also produced the predicted changes in the Ca(2+) dependence of LTD. We conclude that LTD arises from a mechanism that integrates postsynaptic Ca(2+) signals and that this integration may be produced by the positive-feedback cycle.

Dendritic spines linearize the summation of excitatory potentials

In mammalian cortex, most excitatory inputs occur on dendritic spines, avoiding dendritic shafts. Although spines biochemically isolate inputs, nonspiny neurons can also implement biochemical compartmentalization; so, it is possible that spines have an additional function. We have recently shown that the spine neck can filter membrane potentials going into and out of the spine. To investigate the potential function of this electrical filtering, we used two-photon uncaging of glutamate and compared the integration of electrical signals in spines vs. dendritic shafts from basal dendrites of mouse layer 5 pyramidal neurons. Uncaging potentials onto spines summed linearly, whereas potentials on dendritic shafts reduced each other's effect. Linear integration of spines was maintained regardless of the amplitude of the response, distance between spines (as close as < 2 microm), distance of the spines to the soma, dendritic diameter, or spine neck length. Our findings indicate that spines serve as electrical isolators to prevent input interaction, and thus generate a linear arithmetic of excitatory inputs. Linear integration could be an essential feature of cortical and other spine-laden circuits.

The spine neck filters membrane potentials

Dendritic spines receive most synaptic inputs in the forebrain. Their morphology, with a spine head isolated from the dendrite by a slender neck, indicates a potential role in isolating inputs. Indeed, biochemical compartmentalization occurs at spine heads because of the diffusional bottleneck created by the spine neck. Here we investigate whether the spine neck also isolates inputs electrically. Using two-photon uncaging of glutamate on spine heads from mouse layer-5 neocortical pyramidal cells, we find that the amplitude of uncaging potentials at the soma is inversely proportional to neck length. This effect is strong and independent of the position of the spine in the dendritic tree and size of the spine head. Moreover, spines with long necks are electrically silent at the soma, although their heads are activated by the uncaging event, as determined with calcium imaging. Finally, second harmonic measurements of membrane potential reveal an attenuation of somatic voltages into the spine head, an attenuation directly proportional to neck length. We conclude that the spine neck plays an electrical role in the transmission of membrane potentials, isolating synapses electrically.

A comparison between exocytic control mechanisms in adrenal chromaffin cells and a glutamatergic synapse

It has been known since the work of Katz and collaborators in the early 1950s that an increase in intracellular Ca(++) concentration ([Ca(++)]) is the immediate trigger for neurotransmitter release. Later work has shown that, next to Ca(++), many other signaling pathways, particularly via cyclic AMP, modulate the release of both neurotransmitters and hormones. However, regulated secretion is a multistep process and the signaling mechanisms involved act at many stages. Biochemical and traditional electrophysiological techniques very often cannot distinguish whether a change in secretion is caused by regulation of ion channels, vesicle trafficking, or the exocytic process itself. My laboratory has made an effort to dissect the stimulus secretion pathway by developing assays in chromaffin cells (for catecholamine release) and at a glutamatergic central nervous synapse (the calyx of Held, a component of the auditory pathway), which permit the study of secretion in single cells under voltage clamp conditions. This enables us to clearly distinguish between consequences of changes in electrical signaling, from those regarding the process of vesicle recruitment or the process of exocytosis.

Identification of a 2-stage platelet aggregation process mediating shear-dependent thrombus formation

Disturbances of blood flow at sites of atherosclerotic plaque rupture are one of the key pathogenic events promoting platelet activation and arterial thrombus formation. Shear effects of platelets have been extensively investigated in vitro; however, the mechanisms by which shear promotes platelet aggregation in vivo remain incompletely understood. By employing high-resolution imaging techniques to in vitro and in vivo thrombosis models, we demonstrate a unique mechanism initiating shear-dependent platelet aggregation involving aggregate formation between discoid platelets. These discoid platelet aggregates are initially unstable and result from the development of membrane tethers between coadhering platelets. Tether formation involves the adhesive function of GPIb/V/IX and integrin alphaIIbbeta3, and conversion of discoid platelet aggregates into stable aggregates requires released ADP. The efficiency of this process is regulated by 3 independent variables, including the reactivity of the adhesive substrate, the level of shear flow, and the platelet density at the adhesive surface. These studies identify a new mechanism initiating platelet aggregation that is critically influenced by shear, physical proximity between translocating platelets, and membrane tether formation. Moreover, they provide a model to explain how the discoid morphology of platelets facilitates the maintenance of adhesive interactions with thrombogenic surfaces under high shear stress conditions.

DM-nitrophen AM is caged magnesium

DM-nitrophen is a photolabile derivative of EDTA and therefore functions as either caged Ca(2+) or caged Mg(2+). Several papers have been published recently which use the AM ester to load intact cells with DM-nitrophen; under these conditions it is caged Mg(2+). In this commentary, I give a short history of the development and application of caged Ca(2+) probes, explaining why EGTA and BAPTA-based Ca(2+) cages are to be preferred for the photorelease of Ca(2+) under normal intracellular conditions.

Calcium sensitivity of neurotransmitter release differs at phasic and tonic synapses

The efficacy of synaptic transmission varies greatly among synaptic contacts. We have explored the origins of differences between phasic and tonic crustacean neuromuscular junctions. Synaptic boutons of a phasic motor neuron release three orders of magnitude more quanta to a single action potential and show strong depression to a train, whereas tonic synapses are nearly unresponsive to single action potentials and display an immense facilitation. Phasic and tonic synapses display a similar nonlinear dependence on extracellular [Ca2+]. We imposed similar spatially uniform intracellular [Ca2+] ([Ca2+]i) steps in phasic and tonic synapses by photolysis of presynaptic caged calcium. [Ca2+]i was measured fluorometrically while transmitter release was monitored electrophysiologically from single boutons in which the [Ca2+]i was elevated. Phasic synapses released the readily releasable pool (RRP) of vesicles at a much higher rate and with a shorter delay than did tonic synapses. Comparison of several kinetic models of molecular events showed that a difference in Ca2+-sensitive priming of vesicles in the RRP combined with a revision of the kinetic Ca2+-binding sequence to the secretory trigger produced the best fit to the markedly different responses to Ca2+ steps and action potentials and of the characteristic features of synaptic plasticity in phasic and tonic synapses. The results reveal processes underlying one aspect of synaptic diversity that may also regulate changes in synaptic strength during development and learning and memory formation.

The nitrodibenzofuran chromophore: a new caging group for ultra-efficient photolysis in living cells

Photochemical uncaging of bio-active molecules was introduced in 1977, but since then, there has been no substantial improvement in the properties of generic caging chromophores. We have developed a new chromophore, nitrodibenzofuran (NDBF) for ultra-efficient uncaging of second messengers inside cells. Photolysis of a NDBF derivative of EGTA (caged calcium) is about 16-160 times more efficient than photolysis of the most widely used caged compounds (the quantum yield of photolysis is 0.7 and the extinction coefficient is 18,400 M(-1) cm(-1)). Ultraviolet (UV)-laser photolysis of NDBF-EGTA:Ca(2+) rapidly released Ca(2+) (rate of 20,000 s(-1)) and initiated contraction of skinned guinea pig cardiac muscle. NDBF-EGTA has a two-photon cross-section of approximately 0.6 GM and two-photon photolysis induced localized Ca(2+)-induced Ca(2+) release from the sarcoplasmic recticulum of intact cardiac myocytes. Thus, the NDBF chromophore has great promise as a generic and photochemically efficient protecting group for both one- and two-photon uncaging in living cells.

Tuning caged calcium: photolabile analogues of EGTA with improved optical and chelation properties

The physico-chemical properties of several Ca(2+)-selective, photolabile chelators are described. These molecules have been developed as part of an effort to produce a caged Ca(2+) that improved upon the Ca(2+) chelation properties and light absorption capability of nitrophenyl-EGTA (NP-EGTA). Four dimethoxy-ortho-nitrophenyl derivatives of EGTA (called DMNPE-1 through -4), and one analogue of EGTA (DMNPE-5) have been characterized, each of which is bisected upon irradiation. One of these cages has a higher affinity than NP-EGTA: DMNPE-4 has a K(d) for Ca(2+) of 48 nm at pH 7.2 (19 nM at pH 7.4). Furthermore, this cage has a large extinction coefficient of 5120 M(-1)cm(-1) at 350 nm (cf. 975 M(-1)cm(-1) for NP-EGTA). The other physico-chemical properties of DMNPE-4 are: quantum yield of photolysis of 0.09; bipasic Ca(2+) release kinetics (70% released with a rate of about 48,000 s(-1) and 30% at 1.5s(-1)) and photoproducts that bind Ca(2+) with very low affinity (K(d) in the range of 2mM, pH 7.2), hence most of the bound Ca(2+) is released rapidly and efficiently upon photolysis. Thus, DMNPE-4 has a unique combination of properties that make it an extremely effective Ca(2+) cage.

Presynaptic calcium and control of vesicle fusion

Vesicle fusion and transmitter release at synapses is driven by a highly localized Ca2+ signal that rapidly builds up around open Ca2+-channels at and near presynaptic active zones. It has been difficult to estimate the amplitude and the kinetics of this 'microdomain' signal by direct Ca2+-imaging approaches. Recently, Ca2+ uncaging at large CNS synapses, among them the calyx of Held, has shown that the intrinsic cooperativity of Ca2+ in inducing vesicle fusion is high, with 4-5 Ca2+ ions needed to trigger vesicle fusion. Given the Ca2+-sensitivity of vesicle fusion as determined by Ca2+-uncaging, it was found that a surprisingly small (10-25 microM) and brief (<1 ms) local Ca2+ signal is sufficient to achieve the amount, and the kinetics of the physiological transmitter release. The high cooperativity of Ca2+ in inducing vesicle fusion and the non-saturation of the Ca2+-sensor for vesicle fusion renders small changes of the local Ca2+-signal highly effective in changing the release probability; an insight that is important for our understanding of short-term modulation of synaptic strength.

Kinetic properties of DM-nitrophen binding to calcium and magnesium

Caged-Ca(2+) compounds such as nitrophenyl-EGTA (NP-EGTA) and DM-nitrophen (DMn) are extremely useful in biological research, but their use in live cells is hampered by cytoplasmic [Mg(2+)]. We determined the properties of Ca(2+) release from NP-EGTA and DMn by using Oregon green BAPTA-5N to measure changes in [Ca(2+)] after ultraviolet flash photolysis in vitro, with or without Mg(2+) present. A large fraction (65%) of NP-EGTA, which has a negligible Mg(2+) affinity, uncages with a time constant of 10.3 ms, resulting in relatively slow increases in [Ca(2+)]. Uncaging of DMn is considerably faster, but DMn has a significant affinity for Mg(2+) to complicate the uncaging process. With experimentally determined values for the Ca(2+) and Mg(2+) binding/unbinding rates of DMn and NP-EGTA, we built a mathematical model to assess the utility of NP-EGTA and DMn in rapid Ca(2+)-uncaging experiments in the presence of Mg(2+). We discuss the advantages and disadvantages of using each compound under different conditions. To determine the kinetics of Ca(2+) binding to biologically relevant Ca(2+) buffers, such as Ca(2+)-binding proteins, the use of DMn is preferable even in the presence of Mg(2+).

Caged phosphopeptides reveal a temporal role for 14-3-3 in G1 arrest and S-phase checkpoint function

Using classical genetics to study modular phosphopeptide-binding domains within a family of proteins that are functionally redundant is difficult when other members of the domain family compensate for the product of the knocked-out gene. Here we describe a chemical genetics approach that overcomes this limitation by using UV-activatable caged phosphopeptides. By incorporating a caged phosphoserine residue within a consensus motif, these reagents simultaneously and synchronously inactivate all phosphoserine/phosphothreonine-binding domain family members in a rapid and temporally regulated manner. We applied this approach to study the global function of 14-3-3 proteins in cell cycle control. Activation of the caged phosphopeptides by UV irradiation displaced endogenous proteins from 14-3-3-binding, causing premature cell cycle entry, release of G1 cells from interphase arrest and loss of the S-phase checkpoint after DNA damage, accompanied by high levels of cell death. This class of reagents will greatly facilitate molecular dissection of kinase-dependent signaling pathways when applied to other phosphopeptide-binding domains including SH2, Polo-box and tandem BRCT domains.

Live Optical Imaging of Nervous System Development

Although development of the nervous system is inherently a process of dynamic change, until recently it has generally been investigated by inference from static images. However, advances in live optical imaging are now allowing direct observation of growth, synapse formation, and even incipient function in the developing nervous system, at length scales from molecules to cortical regions, and over timescales from milliseconds to months. In this review, we provide technical background and present examples of how these new methods, including confocal and two-photon microscopy, GFP-based markers, and functional indicators, are being applied to provide fresh insight into long-standing questions of neural development.