Imaging Ca2+ concentration changes at the secretory vesicle surface with a recombinant targeted cameleon

The evolution of organellar calcium mapping technologies

Intracellular Ca2+ fluxes are dynamically controlled by the co-involvement of multiple organellar pools of stored Ca2+. Endolysosomes are emerging as physiologically critical, yet underexplored, sources and sinks of intracellular Ca2+. Delineating the role of organelles in Ca2+ signaling has relied on chemical fluorescent probes and electrophysiological strategies. However, the acidic endolysosomal environment presents unique issues, which preclude the use of traditional chemical reporter strategies to map lumenal Ca2+. Here, we broadly address the current state of knowledge about organellar Ca2+ pools. We then outline the application of traditional probes, and their sensing paradigms. We then discuss how a new generation of probes overcomes the limitations of traditional Ca2+probes, emphasizing their ability to offer critical insights into endolysosomal Ca2+, and its feedback with other organellar pools.

The changing view of insulin granule mobility: From conveyor belt to signaling hub

Before the advent of TIRF microscopy the fate of the insulin granule prior to secretion was deduced from biochemical investigations, electron microscopy and electrophysiological measurements. Since Calcium-triggered granule fusion is indisputably necessary to release insulin into the extracellular space, much effort was directed to the measure this event at the single granule level. This has also been the major application of the TIRF microscopy of the pancreatic beta cell when it became available about 20 years ago. To better understand the metabolic modulation of secretion, we were interested to characterize the entirety of the insulin granules which are localized in the vicinity of the plasma membrane to identify the characteristics which predispose to fusion. In this review we concentrate on how the description of granule mobility in the submembrane space has evolved as a result of progress in methodology. The granules are in a state of constant turnover with widely different periods of residence in this space. While granule fusion is associated +with prolonged residence and decreased lateral mobility, these characteristics may not only result from binding to the plasma membrane but also from binding to the cortical actin web, which is present in the immediate submembrane space. While granule age as such affects granule mobility and fusion probability, the preceding functional states of the beta cell leave their mark on these parameters, too. In summary, the submembrane granules form a highly dynamic heterogeneous population and contribute to the metabolic memory of the beta cells.

Insulin secretory granules labelled with phogrin-fluorescent proteins show alterations in size, mobility and responsiveness to glucose stimulation in living β-cells

The intracellular life of insulin secretory granules (ISGs) from biogenesis to secretion depends on their structural (e.g. size) and dynamic (e.g. diffusivity, mode of motion) properties. Thus, it would be useful to have rapid and robust measurements of such parameters in living β-cells. To provide such measurements, we have developed a fast spatiotemporal fluctuation spectroscopy. We calculate an imaging-derived Mean Squared Displacement (iMSD), which simultaneously provides the size, average diffusivity, and anomalous coefficient of ISGs, without the need to extract individual trajectories. Clustering of structural and dynamic quantities in a multidimensional parametric space defines the ISGs' properties for different conditions. First, we create a reference using INS-1E cells expressing proinsulin fused to a fluorescent protein (FP) under basal culture conditions and validate our analysis by testing well-established stimuli, such as glucose intake, cytoskeleton disruption, or cholesterol overload. After, we investigate the effect of FP-tagged ISG protein markers on the structural and dynamic properties of the granule. While iMSD analysis produces similar results for most of the lumenal markers, the transmembrane marker phogrin-FP shows a clearly altered result. Phogrin overexpression induces a substantial granule enlargement and higher mobility, together with a partial de-polymerization of the actin cytoskeleton, and reduced cell responsiveness to glucose stimulation. Our data suggest a more careful interpretation of many previous ISG-based reports in living β-cells. The presented data pave the way to high-throughput cell-based screening of ISG structure and dynamics under various physiological and pathological conditions.

Exploring Protein⁻Protein Interaction in the Study of Hormone-Dependent Cancers

Estrogen receptors promote target gene transcription when they form a dimer, in which two identical (homodimer) or different (heterodimer) proteins are bound to each other. In hormone-dependent cancers, hormone receptor dimerization plays pivotal roles, not only in the pathogenesis or development of the tumors, but also in the development of therapeutic resistance. Protein–protein interactions (PPIs), including dimerization and complex formation, have been also well-known to be required for proteins to exert their functions. The methods which could detect PPIs are genetic engineering (i.e., resonance energy transfer) and/or antibody technology (i.e., co-immunoprecipitation) using cultured cells. In addition, visualization of the target proteins in tissues can be performed using antigen–antibody reactions, as in immunohistochemistry. Furthermore, development of microscopic techniques (i.e., electron microscopy and confocal laser microscopy) has made it possible to visualize intracellular and/or intranuclear organelles. We have recently reported the visualization of estrogen receptor dimers in breast cancer tissues by using the in situ proximity ligation assay (PLA). PLA was developed along the lines of antibody technology development, and this assay has made it possible to visualize PPIs in archival tissue specimens. Localization of PPI in organelles has also become possible using super-resolution microscopes exceeding the resolution limit of conventional microscopes. Therefore, in this review, we summarize the methodologies used for studying PPIs in both cells and tissues, and review the recently reported studies on PPIs of hormones.

TALK-1 channels control β cell endoplasmic reticulum Ca2+ homeostasis

Ca2+ handling by the endoplasmic reticulum (ER) serves critical roles in controlling pancreatic β cell function and becomes perturbed during the pathogenesis of diabetes. ER Ca2+ homeostasis is determined by ion movements across the ER membrane, including K+ flux through K+ channels. We demonstrated that K+ flux through ER-localized TALK-1 channels facilitated Ca2+ release from the ER in mouse and human β cells. We found that β cells from mice lacking TALK-1 exhibited reduced basal cytosolic Ca2+ and increased ER Ca2+ concentrations, suggesting reduced ER Ca2+ leak. These changes in Ca2+ homeostasis were presumably due to TALK-1-mediated ER K+ flux, because we recorded K+ currents mediated by functional TALK-1 channels on the nuclear membrane, which is continuous with the ER. Moreover, overexpression of K+-impermeable TALK-1 channels in HEK293 cells did not reduce ER Ca2+ stores. Reduced ER Ca2+ content in β cells is associated with ER stress and islet dysfunction in diabetes, and islets from TALK-1-deficient mice fed a high-fat diet showed reduced signs of ER stress, suggesting that TALK-1 activity exacerbated ER stress. Our data establish TALK-1 channels as key regulators of β cell ER Ca2+ and suggest that TALK-1 may be a therapeutic target to reduce ER Ca2+ handling defects in β cells during the pathogenesis of diabetes.

Local and regional control of calcium dynamics in the pancreatic islet

Ca²⁺ is the key intracellular regulator of insulin secretion, acting in the β-cell as the ultimate trigger for exocytosis. In response to high glucose, ATP-sensitive K⁺ channel closure and plasma membrane depolarization engage a sophisticated machinery to drive pulsatile cytosolic Ca²⁺ changes. Voltage-gated Ca²⁺ channels, Ca²⁺ -activated K⁺ channels and Na⁺ /Ca²⁺ exchange all play important roles. The use of targeted Ca²⁺ probes has revealed that during each cytosolic Ca²⁺ pulse, uptake of Ca²⁺ by mitochondria, endoplasmic reticulum (ER), secretory granules and lysosomes fine-tune cytosolic Ca²⁺ dynamics and control organellar function. For example, changes in the expression of the Ca²⁺ -binding protein Sorcin appear to provide a link between ER Ca²⁺ levels and ER stress, affecting β-cell function and survival. Across the islet, intercellular communication between highly interconnected "hubs," which act as pacemaker β-cells, and subservient "followers," ensures efficient insulin secretion. Loss of connectivity is seen after the deletion of genes associated with type 2 diabetes (T2D) and follows metabolic and inflammatory insults that characterize this disease. Hubs, which typically comprise ~1%-10% of total β-cells, are repurposed for their specialized role by expression of high glucokinase (Gck) but lower Pdx1 and Nkx6.1 levels. Single cell-omics are poised to provide a deeper understanding of the nature of these cells and of the networks through which they communicate. New insights into the control of both the intra- and intercellular Ca²⁺ dynamics may thus shed light on T2D pathology and provide novel opportunities for therapy.

Exploring cells with targeted biosensors

Cellular signaling networks are composed of multiple pathways, often interconnected, that form complex networks with great potential for cross-talk. Signal decoding depends on the nature of the message as well as its amplitude, temporal pattern, and spatial distribution. In addition, the existence of membrane-bound organelles, which are both targets and generators of messages, add further complexity to the system. The availability of sensors that can localize to specific compartments in live cells and monitor their targets with high spatial and temporal resolution is thus crucial for a better understanding of cell pathophysiology. For this reason, over the last four decades, a variety of strategies have been developed, not only to generate novel and more sensitive probes for ions, metabolites, and enzymatic activity, but also to selectively deliver these sensors to specific intracellular compartments. In this review, we summarize the principles that have been used to target organic or protein sensors to different cellular compartments and their application to cellular signaling.

Pancreatic β-cell identity, glucose sensing and the control of insulin secretion

Insulin release from pancreatic β-cells is required to maintain normal glucose homoeostasis in man and many other animals. Defective insulin secretion underlies all forms of diabetes mellitus, a disease currently reaching epidemic proportions worldwide. Although the destruction of β-cells is responsible for Type 1 diabetes (T1D), both lowered β-cell mass and loss of secretory function are implicated in Type 2 diabetes (T2D). Emerging results suggest that a functional deficiency, involving de-differentiation of the mature β-cell towards a more progenitor-like state, may be an important driver for impaired secretion in T2D. Conversely, at least in rodents, reprogramming of islet non-β to β-cells appears to occur spontaneously in models of T1D, and may occur in man. In the present paper, we summarize the biochemical properties which define the 'identity' of the mature β-cell as a glucose sensor par excellence. In particular, we discuss the importance of suppressing a group of 11 'disallowed' housekeeping genes, including Ldha and the monocarboxylate transporter Mct1 (Slc16a1), for normal nutrient sensing. We then survey the changes in the expression and/or activity of β-cell-enriched transcription factors, including FOXO1, PDX1, NKX6.1, MAFA and RFX6, as well as non-coding RNAs, which may contribute to β-cell de-differentiation and functional impairment in T2D. The relevance of these observations for the development of new approaches to treat T1D and T2D is considered.

Beta cell connectivity in pancreatic islets: a type 2 diabetes target?

Beta cell connectivity describes the phenomenon whereby the islet context improves insulin secretion by providing a three-dimensional platform for intercellular signaling processes. Thus, the precise flow of information through homotypically interconnected beta cells leads to the large-scale organization of hormone release activities, influencing cell responses to glucose and other secretagogues. Although a phenomenon whose importance has arguably been underappreciated in islet biology until recently, a growing number of studies suggest that such cell-cell communication is a fundamental property of this micro-organ. Hence, connectivity may plausibly be targeted by both environmental and genetic factors in type 2 diabetes mellitus (T2DM) to perturb normal beta cell function and insulin release. Here, we review the mechanisms that contribute to beta cell connectivity, discuss how these may fail during T2DM, and examine approaches to restore insulin secretion by boosting cell communication.

Dynamic visualization of calcium-dependent signaling in cellular microdomains

Cells rely on the coordinated action of diverse signaling molecules to sense, interpret, and respond to their highly dynamic external environment. To ensure the specific and robust flow of information, signaling molecules are often spatially organized to form distinct signaling compartments, and our understanding of the molecular mechanisms that guide intracellular signaling hinges on the ability to directly probe signaling events within these cellular microdomains. Ca(2+) signaling in particular owes much of its functional versatility to this type of exquisite spatial regulation. As discussed below, a number of methods have been developed to investigate the mechanistic and functional implications of microdomains of Ca(2+) signaling, ranging from the application of Ca(2+) buffers to the direct and targeted visualization of Ca(2+) signaling microdomains using genetically encoded fluorescent reporters.

Förster resonance energy transfer - an approach to visualize the spatiotemporal regulation of macromolecular complex formation and compartmentalized cell signaling

BACKGROUND

Signaling messengers and effector proteins provide an orchestrated molecular machinery to relay extracellular signals to the inside of cells and thereby facilitate distinct cellular behaviors. Formations of intracellular macromolecular complexes and segregation of signaling cascades dynamically regulate the flow of a biological process.

SCOPE OF REVIEW

In this review, we provide an overview of the development and application of FRET technology in monitoring cyclic nucleotide-dependent signalings and protein complexes associated with these signalings in real time and space with brief mention of other important signaling messengers and effector proteins involved in compartmentalized signaling.

MAJOR CONCLUSIONS

The preciseness, rapidity and specificity of cellular responses indicate restricted alterations of signaling messengers, particularly in subcellular compartments rather than globally. Not only the physical confinement and selective depletion, but also the intra- and inter-molecular interactions of signaling effectors modulate the direction of signal transduction in a compartmentalized fashion. To understand the finer details of various intracellular signaling cascades and crosstalk between proteins and other effectors, it is important to visualize these processes in live cells. Förster Resonance Energy Transfer (FRET) has been established as a useful tool to do this, even with its inherent limitations.

GENERAL SIGNIFICANCE

FRET technology remains as an effective tool for unraveling the complex organization and distribution of various endogenous signaling proteins, as well as the spatiotemporal dynamics of second messengers inside a single cell to distinguish the heterogeneity of cell signaling under normal physiological conditions and during pathological events.

Quantitative measurement of Ca2+ and Zn2+ in mammalian cells using genetically encoded fluorescent biosensors

Genetically encoded, ratiometric, fluorescent biosensors can be used to quantitatively measure intracellular ion concentrations in living cells. We describe important factors to consider when selecting a Ca(2+) or Zn(2+) biosensor, such as the sensor's dissociation constant (K(d')) and its dynamic range. We also discuss the limits of quantitative measurement using these sensors and reasons why a sensor may perform differently in different biological systems or subcellular compartments. We outline protocols for (1) quickly confirming sensor functionality in a new biological system, (2) calibrating a sensor to convert a sensor's FRET ratio to ion concentration, and (3) titrating a sensor in living cells to obtain its K(d') under different experimental conditions.

Generation and characterization of a lysosomally targeted, genetically encoded Ca(2+)-sensor

Distinct spatiotemporal Ca²⁺ signalling events regulate fundamental aspects of eukaryotic cell physiology. Complex Ca²⁺ signals can be driven by release of Ca²⁺ from intracellular organelles that sequester Ca²⁺ such as the ER (endoplasmic reticulum) or through the opening of Ca²⁺-permeable channels in the plasma membrane and influx of extracellular Ca²⁺. Late endocytic pathway compartments including late-endosomes and lysosomes have recently been observed to sequester Ca²⁺ to levels comparable with those found within the ER lumen. These organelles harbour ligand-gated Ca²⁺-release channels and evidence indicates that they can operate as Ca²⁺-signalling platforms. Lysosomes sequester Ca²⁺ to a greater extent than any other endocytic compartment, and signalling from this organelle has been postulated to provide ‘trigger’ release events that can subsequently elicit more extensive Ca²⁺ signals from stores including the ER. In order to investigate lysosomal-specific Ca²⁺ signalling a simple method for measuring lysosomal Ca²⁺ release is essential. In the present study we describe the generation and characterization of a genetically encoded, lysosomally targeted, cameleon sensor which is capable of registering specific Ca²⁺ release in response to extracellular agonists and intracellular second messengers. This probe represents a novel tool that will permit detailed investigations examining the impact of lysosomal Ca²⁺ handling on cellular physiology.

Calcium imaging of the cyclic nucleotide response

Calcium (Ca(2+)) is a key component of the signalling network by which plant cells respond to developmental and environmental signals. A change in guard cell cytosolic free Ca(2+)([Ca(2+)]cyt) is an early event in the response of stomata to both opening and closing stimuli, and cyclic nucleotide-mediated Ca(2+) signalling has been implicated in the regulation of stomatal aperture. A range of techniques have been used to measure [Ca(2+)]cyt in plant cells. Here we describe a potential method for imaging cyclic nucleotide-induced changes in [Ca(2+)]cyt in guard cells using the cameleon ratiometric Ca(2+) reporter protein.

Measurement of cytosolic-free Ca²⁺ in plant tissue

A range of techniques have been used to measure the concentration of cytosolic-free Ca(2+) ([Ca(2+)](cyt)) in plant cells. Fluorescent Ca(2+)-sensitive indicators have been used extensively to measure plant [Ca(2+)](cyt) and a number of techniques are available for loading these into plant cells. Here we describe a method for measuring [Ca(2+)](cyt) in the guard cells of the model plant species Commelina communis by ratio photometry and imaging techniques using the ratiometric fluorescent Ca(2+)-sensitive indicator fura-2.

Using imaging methods to interrogate radiation-induced cell signaling

There is increasing emphasis on the use of systems biology approaches to define radiation-induced responses in cells and tissues. Such approaches frequently rely on global screening using various high throughput 'omics' platforms. Although these methods are ideal for obtaining an unbiased overview of cellular responses, they often cannot reflect the inherent heterogeneity of the system or provide detailed spatial information. Additionally, performing such studies with multiple sampling time points can be prohibitively expensive. Imaging provides a complementary method with high spatial and temporal resolution capable of following the dynamics of signaling processes. In this review, we utilize specific examples to illustrate how imaging approaches have furthered our understanding of radiation-induced cellular signaling. Particular emphasis is placed on protein colocalization, and oscillatory and transient signaling dynamics.

Genetically encoded probes for measurement of intracellular calcium

Small, fluorescent, calcium-sensing molecules have been enormously useful in mapping intracellular calcium signals in time and space, as chapters in this volume attest. Despite their widespread adoption and utility, they suffer some disadvantages. Genetically encoded calcium sensors that can be expressed inside cells by transfection or transgenesis are desirable. The last 10 years have been marked by a rapid evolution in the laboratory of genetically encoded calcium sensors both figuratively and literally, resulting in 11 distinct configurations of fluorescent proteins and their attendant calcium sensor modules. Here, the design logic and performance of this abundant collection of sensors and their in vitro and in vivo use and performance are described. Genetically encoded calcium sensors have proved valuable in the measurement of calcium concentration in cellular organelles, for the most part in single cells in vitro. Their success as quantitative calcium sensors in tissues in vitro and in vivo is qualified, but they have proved valuable in imaging the pattern of calcium signals within tissues in whole animals. Some branches of the calcium sensor evolutionary tree continue to evolve rapidly and the steady progress in optimizing sensor parameters leads to the certain hope that these drawbacks will eventually be overcome by further genetic engineering.

Photoproteins: important new tools in drug discovery

The G protein-coupled receptor (GPCR) family is a major target for drug discovery, and most, if not all, GPCRs can couple to Ca2+ signaling. Consequently, there are a number of cellbased, primary, high-throughput screening (HTS) assays used for drug discovery that assess changes in intracellular Ca2+ as a functional readout of GPCR activation. Historically, changes in intracellular Ca2+ levels have been readily detected using fluorescent dyes that emit light in proportion to changes in intracellular Ca2+ concentration. An alternative approach to indirectly measure changes in Ca2+ concentrations involves the use of recombinantly expressed biosensor photoproteins, of which aequorin is a prototypic example. These biosensors have the advantage that they provide an intense luminescent signal in response to elevations in intracellular Ca2+. This exquisite sensitivity, the high signal-to-noise ratios, and the ability to target expression to discrete subcellular sites (in order to detect Ca2+ microdomains) have made photoproteins a principal choice in a wide range of GPCR drug discovery programs. Photoproteins are also finding increasing use in detecting activation of other molecular target classes such as ligand-gated ion channels and transporters. This review focuses upon the use of calcium photoproteins principally for use in GPCR drug discovery and HTS.

Gene silencing of phogrin unveils its essential role in glucose-responsive pancreatic beta-cell growth

OBJECTIVE

Phogrin and IA-2, autoantigens in insulin-dependent diabetes, have been shown to be involved in insulin secretion in pancreatic beta-cells; however, implications at a molecular level are confusing from experiment to experiment. We analyzed biological functions of phogrin in beta-cells by an RNA interference technique.

RESEARCH DESIGN AND METHODS

Adenovirus-mediated expression of short hairpin RNA specific for phogrin (shPhogrin) was conducted using cultured beta-cell lines and mouse islets. Both glucose-stimulated insulin secretion and cell proliferation rate were determined in the phogrin-knockdown cells. Furthermore, protein expression was profiled in these cells. To see the binding partner of phogrin in beta-cells, coimmunoprecipitation analysis was carried out.

RESULTS

Adenoviral expression of shPhogrin efficiently decreased its endogenous expression in pancreatic beta-cells. Silencing of phogrin in beta-cells abrogated the glucose-mediated mitogenic effect, which was accompanied by a reduction in the level of insulin receptor substrate 2 (IRS2) protein, without any changes in insulin secretion. Phogrin formed a complex with insulin receptor at the plasma membrane, and their interaction was promoted by high-glucose stimulation that in turn led to stabilization of IRS2 protein. Corroboratively, phogrin knockdown had no additional effect on the proliferation of beta-cell line derived from the insulin receptor-knockout mouse.

CONCLUSIONS

Phogrin is involved in beta-cell growth via regulating stability of IRS2 protein by the molecular interaction with insulin receptor. We propose that phogrin and IA-2 function as an essential regulator of autocrine insulin action in pancreatic beta-cells.

A rare mutation in ABCC8/SUR1 leading to altered ATP-sensitive K+ channel activity and beta-cell glucose sensing is associated with type 2 diabetes in adults

OBJECTIVE

ATP-sensitive K(+) channels (K(ATP) channels) link glucose metabolism to the electrical activity of the pancreatic beta-cell to regulate insulin secretion. Mutations in either the Kir6.2 or sulfonylurea receptor (SUR) 1 subunit of the channel have previously been shown to cause neonatal diabetes. We describe here an activating mutation in the ABCC8 gene, encoding SUR1, that is associated with the development of type 2 diabetes only in adults.

RESEARCH DESIGN AND METHODS

Recombinant K(ATP) channel subunits were expressed using pIRES2-based vectors in human embryonic kidney (HEK) 293 or INS1(832/13) cells and the subcellular distribution of c-myc-tagged SUR1 channels analyzed by confocal microscopy. K(ATP) channel activity was measured in inside-out patches and plasma membrane potential in perforated whole-cell patches. Cytoplasmic [Ca(2+)] was imaged using Fura-Red.

RESULTS

A mutation in ABCC8/SUR1, leading to a Y356C substitution in the seventh membrane-spanning alpha-helix, was observed in a patient diagnosed with hyperglycemia at age 39 years and in two adult offspring with impaired insulin secretion. Single K(ATP) channels incorporating SUR1-Y356C displayed lower sensitivity to MgATP (IC(50) = 24 and 95 micromol/l for wild-type and mutant channels, respectively). Similar effects were observed in the absence of Mg(2+), suggesting an allosteric effect via associated Kir6.2 subunits. Overexpression of SUR1-Y356C in INS1(832/13) cells impaired glucose-induced cell depolarization and increased in intracellular free Ca(2+) concentration, albeit more weakly than neonatal diabetes-associated SUR1 mutants.

CONCLUSIONS

An ABCC8/SUR1 mutation with relatively minor effects on K(ATP) channel activity and beta-cell glucose sensing causes diabetes in adulthood. These data suggest a close correlation between altered SUR1 properties and clinical phenotype.

How to measure Ca2+ in cellular organelles?

The review will aim to briefly summarise information on calcium measurements in cellular organelles with emphases on studies conducted in live cells using optical probes. When appropriate we will try to compare the effectiveness of different indicators for intraorganellar calcium measurements. We will consider calcium measurements in endoplasmic reticulum, Golgi apparatus, endosomes/lysosomes, nucleoplasm, nuclear envelope, mitochondria and secretory granules.

Characterization and subcellular targeting of GCaMP-type genetically-encoded calcium indicators

Genetically-encoded calcium indicators (GECIs) hold the promise of monitoring [Ca²⁺] in selected populations of neurons and in specific cellular compartments. Relating GECI fluorescence to neuronal activity requires quantitative characterization. We have characterized a promising new genetically-encoded calcium indicator—GCaMP2—in mammalian pyramidal neurons. Fluorescence changes in response to single action potentials (17±10% ΔF/F [mean±SD]) could be detected in some, but not all, neurons. Trains of high-frequency action potentials yielded robust responses (302±50% for trains of 40 action potentials at 83 Hz). Responses were similar in acute brain slices from in utero electroporated mice, indicating that long-term expression did not interfere with GCaMP2 function. Membrane-targeted versions of GCaMP2 did not yield larger signals than their non-targeted counterparts. We further targeted GCaMP2 to dendritic spines to monitor Ca²⁺ accumulations evoked by activation of synaptic NMDA receptors. We observed robust ΔF/F responses (range: 37%–264%) to single spine uncaging stimuli that were correlated with NMDA receptor currents measured through a somatic patch pipette. One major drawback of GCaMP2 was its low baseline fluorescence. Our results show that GCaMP2 is improved from the previous versions of GCaMP and may be suited to detect bursts of high-frequency action potentials and synaptic currents in vivo.

Origins of the regulated secretory pathway

Modes of transport of soluble (or luminal) secretory proteins synthesized in the endoplasmic reticulum (ER) could be divided into two groups. The socalled constitutive secretory pathway (CSP) is common to all eukaryotic cells, constantly delivering constitutive soluble secretory proteins (CSSPs) linked to the rate of protein synthesis but largely independent of external stimuli. In regulated secretion, protein is sorted from the Golgi into storage/secretory granules (SGs) whose contents are released when stimuli trigger their final fusion with the plasma membrane (Hannah et al. 1999).

MicroRNA-124a regulates Foxa2 expression and intracellular signaling in pancreatic beta-cell lines

MicroRNAs (miRNAs) are short non-coding RNAs that have been implicated in fine-tuning gene regulation, although the precise roles of many are still unknown. Pancreatic development is characterized by the complex sequential expression of a gamut of transcription factors. We have performed miRNA expression profiling at two key stages of mouse embryonic pancreas development, e14.5 and e18.5. miR-124a2 expression was strikingly increased at e18.5 compared with e14.5, suggesting a possible role in differentiated beta-cells. Among the potential miR-124a gene targets identified by biocomputation, Foxa2 is known to play a role in beta-cell differentiation. To evaluate the impact of miR-124a2 on gene expression, we overexpressed or down-regulated miR-124a2 in MIN6 beta-cells. As predicted, miR-124a2 regulated Foxa2 gene expression, and that of its downstream target, pancreatic duodenum homeobox-1 (Pdx-1). Foxa2 has been described as a master regulator of pancreatic development and also of genes involved in glucose metabolism and insulin secretion, including the ATP-sensitive K(+) (K(ATP)) channel subunits, Kir6.2 and Sur-1. Correspondingly, miR-124a2 overexpression decreased, and anti-miR-124a2 increased Kir6.2 and Sur-1 mRNA levels. Moreover, miR-124a2 modified basal and glucose- or KCl-stimulated intracellular free Ca(2+) concentrations in single MIN6 and INS-1 (832/13) beta-cells, without affecting the secretion of insulin or co-transfected human growth hormone, consistent with an altered sensitivity of the beta-cell exocytotic machinery to Ca(2+). In conclusion, whereas the precise role of microRNA-124a2 in pancreatic development remains to be deciphered, we identify it as a regulator of a key transcriptional protein network in beta-cells responsible for modulating intracellular signaling.

K+-induced ion-exchanges trigger trypsin activation in pancreas acinar zymogen granules

Trypsin premature activation has been thought to be a key event in the initiation phase of acute pancreatitis. Here we test a hypothesis that a sustained increase of cytosolic Ca(2+) concentration ([Ca(2+)](C)) can trigger K(+) influx into pancreas acinar zymogen granules (ZGs) via a Ca(2+)-activated K(+) channel (K(Ca)), and this influx of K(+) then mobilizes bound-Ca(2+) by K(+)/Ca(2+) ion-exchange to increase free Ca(2+) concentration in the ZGs ([Ca(2+)](G)) and release bound-H(+) by K(+)/H(+) ion-exchange to decrease the pH in ZGs (pH(G)). Both the increase of [Ca(2+)](G) and the decrease of pH(G) will facilitate trypsinogen autoactivation and stabilize active trypsin inside ZGs that could lead to acute pancreatitis. The experimental results are consistent with our hypothesis, suggesting that K(+) induced ion-exchanges play a critical role in the initiation of trypsin premature activation in ZGs.

Rapid three-dimensional imaging of individual insulin release events by Nipkow disc confocal microscopy

Minute-to-minute control of the release of insulin by pancreatic beta-cells in response to glucose or other stimuli requires the precise delivery of large dense-core vesicles to the plasma membrane and regulated exocytosis. At present, the precise spatial organization at the cell surface and the nature of these events ('transient' versus 'full fusion') are debated. In order to monitor secretory events simultaneously over most of the surface of clusters of single MIN6 beta-cells, we have expressed recombinant neuropeptide Y-Venus (an enhanced and vesicle-targeted form of yellow fluorescent protein) as an insulin surrogate. Individual exocytotic events were monitored using Nipkow spinning disc confocal microscopy, with acquisition of a three-dimensional complete image (eight to twelve confocal slices) in <1 s, in response to cell depolarization. Corroborating earlier studies using TIRF (total internal reflection fluorescence) microscopy, this approach indicates that events occur with roughly equal probability over the entire cell surface, with only minimal clustering in individual areas, and provides no evidence for multiple events at the same site. Nipkow disc confocal imaging may thus provide a useful tool to determine whether event types occur at different sites at the cell surface and to explore the role of endocytic proteins including dynamin-1 and -2 in terminating individual exocytotic events.

Pro-survival role of gelsolin in mouse beta-cells

We have previously shown that the Ca(2+)-dependent actin-severing protein gelsolin plays an important role in regulated insulin secretion. The aim of this study was to determine the role of gelsolin in beta-cell survival as it has been shown to play a dual role in apoptosis in other cell types. MIN6 subclones B1 and C3, shown previously to express gelsolin at different levels (B1>>C3 cells), were used for this purpose. We demonstrate that B1 cells have lower levels of apoptosis and active caspase-3 when compared with C3 cells, in both standard (25 mmol/l glucose and 15% FCS) and deprived (5 mmol/l glucose and 1% FCS) conditions. Overexpression of gelsolin resulted in a decrease in the percentage of terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL)(+) and active caspase-3(+) cells. Conversely, knockdown of gelsolin by RNA interference in B1 cells caused an increase in the number of TUNEL(+) and active caspase-3(+) cells. Finally, the anti-apoptotic role of gelsolin was confirmed in purified primary mouse beta-cells where overexpression of gelsolin resulted in a decrease in the percentage of TUNEL(+) cells. In summary, our results show for the first time that gelsolin plays a pro-survival role in pancreatic beta-cells.

Ca2+ microdomains and the control of insulin secretion

Nutrient-induced increases in intracellular free Ca(2+) concentrations are the key trigger for insulin release from pancreatic islet beta-cells. These Ca(2+) changes are tightly regulated temporally, occurring as Ca(2+) influx-dependent baseline oscillations. We explore here the concept that locally high [Ca(2+)] concentrations (i.e. Ca(2+) microdomains) may control exocytosis via the recruitment of key effector proteins to sites of exocytosis. Importantly, recent advances in the development of organelle- and membrane-targeted green fluorescent protein (GFP-) or aequorin-based Ca(2+) indicators, as well as in rapid imaging techniques, are providing new insights into the potential role of these Ca(2+) microdomains in beta-cells. We summarise here some of the evidence indicating that Ca(2+) microdomains beneath the plasma membrane and at the surface of large dense core vesicles may be important in the normal regulation of insulin secretion, and may conceivably contribute to "ATP-sensitive K(+)-channel independent" effects of glucose. We also discuss evidence that, in contrast to certain non-excitable cells, direct transfer of Ca(2+) from the ER to mitochondria via localised physical contacts between these organelles is relatively less important for efficient mitochondrial Ca(2+) uptake in beta-cells. Finally, we discuss evidence from single cell imaging that increases in cytosolic Ca(2+) are not required for the upstroke of oscillations in mitochondrial redox state, but may underlie the reoxidation process.

Calcium microdomains in regulated exocytosis

Katz and co-workers showed that Ca(2+) triggers exocytosis. The existence of sub-micrometer domains of greater than 100 microM [Ca(2+)](i) was postulated on theoretical grounds. Using a modified, low-affinity aequorin, Llinas et al. were the first to demonstrate the existence of Ca(2+) 'microdomains' in squid presynaptic terminals. Over the past several years, it has become clear that individual Ca(2+) nano- and microdomains forming around the mouth of voltage-gated Ca(2+) channels ascertain the tight coupling of fast synaptic vesicle release to membrane depolarization by action potentials. Recent work has established different geometric arrangements of vesicles and Ca(2+) channels at different central synapses and pointed out the role of Ca(2+) syntillas - localized, store operated Ca(2+) signals - in facilitation and spontaneous release. The coupling between Ca(2+) increase and evoked exocytosis is more sluggish in peripheral terminals and neuroendocrine cells, where channels are less clustered and Ca(2+) comes from different sources, including Ca(2+) influx via the plasma membrane and the mobilization of Ca(2+) from intracellular stores. Finally, also non- (electrically) excitable cells display highly localized Ca(2+) signaling domains. We discuss in particular the organization of structural microdomains of Bergmann glia, specialized astrocytes of the cerebellum that have only recently been considered as secretory cells. Glial microdomains are the spatial substrate for functionally segregated Ca(2+) signals upon metabotropic activation. Our review emphasizes the large diversity of different geometric arrangements of vesicles and Ca(2+) sources, leading to a wide spectrum of Ca(2+) signals triggering release.

Visualizing calcium signaling in cells by digitized wide-field and confocal fluorescent microscopy

Calcium (Ca2+) is a fundamentally important component of cellular signal transduction. Dynamic changes in the concentration of Ca2+ ([Ca2+]) in the cytoplasm and within organelles are tightly controlled and regulate a diverse array of biological activities, including fertilization, cell division, gene expression, cellular metabolism, protein biosynthesis, secretion, muscle contraction, intercellular communication, and cell death. Measurement of intracellular [Ca2+] is essential to understanding the role of Ca2+ and for defining the underlying regulatory mechanisms in any cellular process. A broad range of synthetic and biosynthetic fluorescent Ca2+ sensors are available that enable the visualization and quantification of subcellular spatio-temporal [Ca2+] gradients. This chapter describes the application of wide-field digitized video fluorescence microfluorometry and confocal microscopy to quantitatively image Ca2+ in cells with high temporal and spatial resolution.

Insulin secretion in health and disease: genomics, proteomics and single vesicle dynamics

Defective insulin secretion from pancreatic islet β-cells is a sine qua non of Type II (non-insulin-dependent) diabetes. Digital imaging analysis of the nanomechanics of individual exocytotic events, achieved using total internal reflection fluorescence microscopy, has allowed us to demonstrate that insulin is released via transient or ‘cavicapture’ events whereby the vesicle and plasma membranes fuse transiently and reversibly. Such studies reveal that an increase in the number of abortive fusion events contributes to defective insulin secretion in in vitro models of Type II diabetes. Complementary analyses of genome-wide changes in β-cell gene expression, at both the mRNA and protein levels, are now facilitating the identification of key molecular players whose altered expression may contribute to the secretory defects in the diabetic β-cell.

Regulation of two insulin granule populations within the reserve pool by distinct calcium sources

Insulin granule trafficking is a key step of glucose-stimulated insulin secretion from pancreatic beta cells. Using quantitative live cell imaging, we examined insulin granule movements within the reserve pool upon secretory stimulation in betaTC3 cells. For this study, we developed a custom image analysis program that permitted automatic tracking of the individual motions of over 20,000 granules. This analysis of a large sample size enabled us to study micro-populations of granules that were not quantifiable in previous studies. While over 90% of the granules depend on Ca2+ efflux from the endoplasmic reticulum for their mobilization, a small and fast-moving population of granules responds to extracellular Ca2+ influx after depolarization of the plasma membrane. We show that this differential regulation of the two granule populations is consistent with localized Ca2+ signals, and that the cytoskeletal network is involved in both types of granule movement. The fast-moving granules are correlated temporally and spatially to the replacement of the secreted insulin granules, which supports the hypothesis that these granules are responsible for replenishing the readily releasable pool. Our study provides a model by which glucose and other secretory stimuli can regulate the readily releasable pool through the same mechanisms that regulate insulin secretion.

Visualization of local Ca2+ dynamics with genetically encoded bioluminescent reporters

Measurements of local Ca2+ signalling at different developmental stages and/or in specific cell types is important for understanding aspects of brain functioning. The use of light excitation in fluorescence imaging can cause phototoxicity, photobleaching and auto-fluorescence. In contrast, bioluminescence does not require the input of radiative energy and can therefore be measured over long periods, with very high temporal resolution. Aequorin is a genetically encoded Ca(2+)-sensitive bioluminescent protein, however, its low quantum yield prevents dynamic measurements of Ca2+ responses in single cells. To overcome this limitation, we recently reported the bi-functional Ca2+ reporter gene, GFP-aequorin (GA), which was developed specifically to improve the light output and stability of aequorin chimeras [V. Baubet, et al., (2000) PNAS, 97, 7260-7265]. In the current study, we have genetically targeted GA to different microdomains important in synaptic transmission, including to the mitochondrial matrix, endoplasmic reticulum, synaptic vesicles and to the postsynaptic density. We demonstrate that these reporters enable 'real-time' measurements of subcellular Ca2+ changes in single mammalian neurons using bioluminescence. The high signal-to-noise ratio of these reporters is also important in that it affords the visualization of Ca2+ dynamics in cell-cell communication in neuronal cultures and tissue slices. Further, we demonstrate the utility of this approach in ex-vivo preparations of mammalian retina, a paradigm in which external light input should be controlled. This represents a novel molecular imaging approach for non-invasive monitoring of local Ca2+ dynamics and cellular communication in tissue or whole animal studies.

Examining intracellular organelle function using fluorescent probes: from animalcules to quantum dots

Fluorescence microscopy imaging has become one of the most useful techniques to assess the activity of individual cells, subcellular trafficking of signals to and between organelles, and to appreciate how organelle function is regulated. The past 2 decades have seen a tremendous advance in the rational design and development in the nature and selectivity of probes to serve as reporters of the intracellular environment in live cells. These probes range from small organic fluorescent molecules to fluorescent biomolecules and photoproteins ingeniously engineered to follow signaling traffic, sense ionic and nonionic second messengers, and report various kinase activities. These probes, together with recent advances in imaging technology, have enabled significantly enhanced spatial and temporal resolution. This review summarizes some of these developments and their applications to assess intracellular organelle function.

Visualising insulin secretion. The Minkowski Lecture 2004

Insulin secretion from pancreatic islet beta cells is a tightly regulated process, under the close control of blood glucose concentrations, neural inputs and circulating hormones. Defects in glucose-triggered insulin secretion, possibly exacerbated by a decrease in beta cell mass, are ultimately responsible for the development of type 2 diabetes. A full understanding of the mechanisms by which glucose and other nutrients trigger insulin secretion will probably be essential to allow for the development of new therapies of type 2 diabetes and for the derivation of "artificial" beta cells from embryonic stem cells as a treatment for type 1 diabetes. I focus here on recent developments in our understanding of beta cell glucose sensing, achieved in part through the development of recombinant targeted probes (luciferase, green fluorescent protein) that allow islet beta cell metabolism and Ca(2+) handling to be imaged in situ in the intact islet with single cell resolution. Combined with classical biochemistry, these techniques show that the beta cell is uniquely poised, thanks to the expression of low levels of lactate dehydrogenase and plasma membrane lactate/monocarboxylate transporters, to channel glucose carbons towards oxidative metabolism, ATP synthesis and inhibition of AMP-activated protein kinase, a newly defined regulator of insulin release. I also discuss the molecular basis of the recruitment of secretory vesicles to the cell surface, analysed by the use of new imaging techniques including total internal reflection of fluorescence, as well as the "nanomechanics" of the exocytotic event itself.

Use of chimeric fluorescent proteins and fluorescence resonance energy transfer to monitor cellular responses

In recent years, the development of new technologies based on the green fluorescent protein and fluorescence resonance energy transfer has introduced a new perspective in the study of cell biology. Real-time imaging of fluorescent biosensors has made it possible to directly visualize individual molecular events as they happen in intact, live cells, providing important and original insights for our understanding of biologically relevant problems. This review discusses some essential methodological aspects concerning the generation and use of fluorescence resonance energy transfer-based biosensors and presents selected examples of specific applications that highlight the power of this technology.

Involvement of Per-Arnt-Sim (PAS) kinase in the stimulation of preproinsulin and pancreatic duodenum homeobox 1 gene expression by glucose

Per-Arnt-Sim (PAS) domain-containing kinases are common in prokaryotes, but a mammalian counterpart has only recently been described. Although the PAS domain of the mammalian PAS kinase (PASK) is closely related to the bacterial oxygen sensor FixL, it is unclear whether PASK activity is changed in mammalian cells in response to nutrients and might therefore contribute to signal transduction by these or other stimuli. Here, we show that elevated glucose concentrations rapidly increase PASK activity in pancreatic islet beta cells, an event followed by the accumulation of both PASK mRNA and protein. Demonstrating a physiological role for PASK activation, comicroinjection into clonal beta cells of cDNA encoding wild-type PASK, or PASK protein itself, mimics the induction of preproinsulin promoter activity by high glucose concentrations. Conversely, anti-PASK antibodies block promoter activation by the sugar, and the silencing of PASK expression by RNA interference suppresses the up-regulation by glucose of preproinsulin and pancreatic duodenum homeobox 1 gene expression, without affecting glucose-induced changes in the levels of mRNAs encoding glucokinase or uncoupling protein 2. We conclude that PASK is an important metabolic sensor in nutrient-sensitive mammalian cells and plays an unexpected role in the regulation of key genes involved in maintaining the differentiated phenotype of pancreatic beta cells.

Role for plasma membrane-related Ca2+-ATPase-1 (ATP2C1) in pancreatic beta-cell Ca2+ homeostasis revealed by RNA silencing

Changes in intracellular Ca(2+) concentration play a key role in the regulation of insulin secretion by glucose and other secretagogues. Here, we explore the importance of the secretory pathway Ca(2+)-ATPase, plasma membrane-related Ca(2+)-ATPase-1 (PMR1; human orthologue ATP2C1) in intracellular Ca(2+) homeostasis in pancreatic islet beta-cells. Endogenous PMR1 mRNA and protein were detected in both isolated rat islets and beta-cell-derived lines (MIN6 and INS1). Subcellular fractionation of the cell lines revealed PMR1 immunoreactivity in both microsomal and dense-core secretory vesicle-enriched fractions. Correspondingly, depletion of cellular PMR1 with small interfering RNAs inhibited Ca(2+) uptake into the endoplasmic reticulum and secretory vesicles by approximately 20%, as assessed using organelle-targeted aequorins in permeabilized INS1 cells. In intact cells, PMR1 depletion markedly enhanced flux though L-type Ca(2+) channels and augmented glucose-stimulated, but not basal, insulin secretion. Whereas average cytosolic [Ca(2+)] increases in response to 30.0 mmol/l glucose were unaffected by PMR1 depletion, [Ca(2+)] oscillation shape, duration, and decay rate in response to glucose plus tetraethylammonium were modified in PMR1-depleted single cells, imaged using fluo-3-acetoxymethylester. PMR1 thus plays an important role, which is at least partially nonoverlapping with that of sarco(endo-)plasmic reticulum Ca(2+)-ATPases, in the control of beta-cell Ca(2+) homeostasis and insulin secretion.

Measurements of the free luminal ER Ca(2+) concentration with targeted "cameleon" fluorescent proteins

The free ER Ca(2+) concentration, [Ca(2+)](ER), is a key parameter that determines both the spatio-temporal pattern of Ca(2+) signals as well as the activity of ER-resident enzymes. Obtaining accurate, time-resolved measurements of the Ca(2+) activity within the ER is thus critical for our understanding of cell signaling. Such measurements, however, are particularly challenging given the highly dynamic nature of Ca(2+) signals, the complex architecture of the ER, and the difficulty of addressing probes specifically into the ER lumen. Prompted by these challenges, a number of ingenious approaches have been developed over the last years to measure ER Ca(2+) by optical means. The two main strategies used to date are Ca(2+)-sensitive synthetic dyes trapped into organelles and genetically encoded probes, based either on the photoprotein aequorin or on the green fluorescent protein (GFP). The GFP-based Ca(2+) indicators comprise the camgaroo and pericam probes based on a circularly permutated GFP, and the cameleon probes, which rely on the fluorescence resonance energy transfer (FRET) between two GFP mutants of different colors. Each approach offers unique advantages and suffers from specific drawbacks. In this review, we will discuss the advantages and pitfalls of using the genetically encoded "cameleon" Ca(2+) indicators for ER Ca(2+) measurements.

Essential role of Ca2+/calmodulin in Early Endosome Antigen-1 localization

Ca2+ is an essential requirement in membrane fusion, acting through binding proteins such as calmodulin (CaM). Ca2+/CaM is required for early endosome fusion in vitro, however, the molecular basis for this requirement is unknown. An additional requirement for endosome fusion is the protein Early Endosome Antigen 1 (EEA1), and its recruitment to the endosome depends on phosphatidylinositol 3-phosphate [PI(3)P] and the Rab5 GTPase. Herein, we demonstrate that inhibition of Ca2+/CaM, by using either chemical inhibitors or specific antibodies directed to CaM, results in a profound inhibition of EEA1 binding to endosomal membranes both in live cells and in vitro. The concentration of Ca2+/CaM inhibitors required for a full dissociation of EEA1 from endosomal membranes had no effect on the activity of phosphatidylinositol 3-kinases or on endogenous levels of PI(3)P. However, the interaction of EEA1 with liposomes containing PI(3)P was decreased by Ca2+/CaM inhibitors. Thus, Ca2+/CaM seems to be required for the stable interaction of EEA1 with endosomal PI(3)P, perhaps by directly or indirectly stabilizing the quaternary organization of the C-terminal FYVE domain of EEA1. This requirement is likely to underlie at least in part the essential role of Ca2+/CaM in endosome fusion.

Looking forward to seeing calcium

From molecules, single cells and tissues to whole organisms, our insights into Ca2+ signalling and the corresponding physiological phenomena are growing exponentially. Here, we describe the improvements that have been made in the development of the probes and instrumentation that are used for Ca2+ imaging and the expanding applications of Ca2+ imaging in basic and applied research.

The Theodore Bücher lecture. Investigating signal transduction with genetically encoded fluorescent probes

Ca2+ and cAMP are ubiquitous second messengers in eukaryotes and control numerous physiological responses ranging from fertilization to cell death induction. To distinguish between these different responses, their subtle regulation in time, space and amplitude is needed. Therefore, the characterization of the signalling process requires measurement of second messengers with tools of precise localization, high dynamic range and as little disturbance of cell physiology as possible. Recently, fluorescent proteins of marine jellyfish have given rise to a set of genetically encoded biosensors which fulfil these criteria and which have already led to important new insights into the subcellular handling of Ca2+ and cAMP. The use of these probes in combination with new microscopical methods such as two-photon microscopy now enables researchers to study second messenger signalling in intact tissues. In this review, the genetically encoded measurement probes and their origin are briefly introduced and some recent insights into the spatio-temporal complexity of both Ca2+ and cAMP signalling obtained with these tools are discussed.

Ryanodine receptor type I and nicotinic acid adenine dinucleotide phosphate receptors mediate Ca2+ release from insulin-containing vesicles in living pancreatic beta-cells (MIN6)

We have demonstrated recently (Mitchell, K. J., Pinton, P., Varadi, A., Tacchetti, C., Ainscow, E. K., Pozzan, T., Rizzuto, R., and Rutter, G. A. (2001) J. Cell Biol. 155, 41-51) that ryanodine receptors (RyR) are present on insulin-containing secretory vesicles. Here we show that pancreatic islets and derived beta-cell lines express type I and II, but not type III, RyRs. Purified by subcellular fractionation and membrane immuno-isolation, dense core secretory vesicles were found to possess a similar level of type I RyR immunoreactivity as Golgi/endoplasmic reticulum (ER) membranes but substantially less RyR II than the latter. Monitored in cells expressing appropriately targeted aequorins, dantrolene, an inhibitor of RyR I channels, elevated free Ca(2+) concentrations in the secretory vesicle compartment from 40.1 +/- 6.7 to 90.4 +/- 14.8 microm (n = 4, p < 0.01), while having no effect on ER Ca(2+) concentrations. Furthermore, nicotinic acid adenine dinucleotide phosphate (NAADP), a novel Ca(2+)-mobilizing agent, decreased dense core secretory vesicle but not ER free Ca(2+) concentrations in permeabilized MIN6 beta-cells, and flash photolysis of caged NAADP released Ca(2+) from a thapsigargin-insensitive Ca(2+) store in single MIN6 cells. Because dantrolene strongly inhibited glucose-stimulated insulin secretion (from 3.07 +/- 0.51-fold stimulation to no significant glucose effect; n = 3, p < 0.01), we conclude that RyR I-mediated Ca(2+)-induced Ca(2+) release from secretory vesicles, possibly potentiated by NAADP, is essential for the activation of insulin secretion.

Visualization of the spatial and temporal dynamics of intracellular signaling

In recent years, our ability to unravel the finer details of intracellular signaling has improved remarkably. Technological innovations resulting from the introduction of green fluorescent protein (GFP) have played a significant role in these advances. Fluorescent indicators allow us to visualize events within a cell in real-time and space. This review focuses on indicators that use GFP-based fluorescence resonance energy transfer (FRET) technologies and discusses how these methodologies have given insights into biological questions relating to the spatiotemporal patterns of signaling by cAMP, calcium, receptor tyrosine kinases, and other molecules.

Monitoring of free calcium in the neuronal endoplasmic reticulum: an overview of modern approaches

The concentration of free calcium within the lumen of the endoplasmic reticulum ([Ca2+]L) fluctuates between 100 and 1000 microM. High [Ca2+]L provides an electro-driving force for Ca2+ release and supports high Ca2+ diffusion rate within the endoplasmic reticulum lumen. Fluctuations in [Ca2+]L also regulate numerous chaperones, responsible for postranslational protein processing. Thus, [Ca2+]L integrates various signalling events and establishes a link between fast signalling, associated with the endoplasmic reticulum Ca2+release/uptake, and long-lasting adaptive responses relying primarily on the regulation of protein synthesis. This paper overviews modern approaches for the direct monitoring of [Ca2+]L which rely on three classes of low-affinity Ca2+ probes: ER-targeted aequorin, synthetic fluorescent Ca2+ dyes and GFP-based ER-targeted Ca2+ probes. These techniques, especially as applied to neurones, may substantially widen our appreciation of the endoplasmic reticulum as a universal signalling organelle.

Dynamics of glucose-induced membrane recruitment of protein kinase C beta II in living pancreatic islet beta-cells

The mechanisms by which glucose may affect protein kinase C (PKC) activity in the pancreatic islet beta-cell are presently unclear. By developing adenovirally expressed chimeras encoding fusion proteins between green fluorescent protein and conventional (betaII), novel (delta), or atypical (zeta) PKCs, we show that glucose selectively alters the subcellular localization of these enzymes dynamically in primary islet and MIN6 beta-cells. Examined by laser scanning confocal or total internal reflection fluorescence microscopy, elevated glucose concentrations induced oscillatory translocations of PKCbetaII to spatially confined regions of the plasma membrane. Suggesting that increases in free cytosolic Ca(2+) concentrations ([Ca(2+)](c)) were primarily responsible, prevention of [Ca(2+)](c) increases with EGTA or diazoxide completely eliminated membrane recruitment, whereas elevation of cytosolic [Ca(2+)](c) with KCl or tolbutamide was highly effective in redistributing PKCbetaII both to the plasma membrane and to the surface of dense core secretory vesicles. By contrast, the distribution of PKCdelta.EGFP, which binds diacylglycerol but not Ca(2+), was unaffected by glucose. Measurement of [Ca(2+)](c) immediately beneath the plasma membrane with a ratiometric "pericam," fused to synaptic vesicle-associated protein-25, revealed that depolarization induced significantly larger increases in [Ca(2+)](c) in this domain. These data demonstrate that nutrient stimulation of beta-cells causes spatially and temporally complex changes in the subcellular localization of PKCbetaII, possibly resulting from the generation of Ca(2+) microdomains. Localized changes in PKCbetaII activity may thus have a role in the spatial control of insulin exocytosis.