Targeting aequorin to the endoplasmic reticulum of living cells

Temporally multiplexed imaging of dynamic signaling networks in living cells

Molecular signals interact in networks to mediate biological processes. To analyze these networks, it would be useful to image many signals at once, in the same living cell, using standard microscopes and genetically encoded fluorescent reporters. Here, we report temporally multiplexed imaging (TMI), which uses genetically encoded fluorescent proteins with different clocklike properties-such as reversibly photoswitchable fluorescent proteins with different switching kinetics-to represent different cellular signals. We linearly decompose a brief (few-second-long) trace of the fluorescence fluctuations, at each point in a cell, into a weighted sum of the traces exhibited by each fluorophore expressed in the cell. The weights then represent the signal amplitudes. We use TMI to analyze relationships between different kinase activities in individual cells, as well as between different cell-cycle signals, pointing toward broad utility throughout biology in the analysis of signal transduction cascades in living systems.

Transgenic mice encoding modern imaging probes: Properties and applications

Modern biology is increasingly reliant on optical technologies, including visualization and longitudinal monitoring of cellular processes. The major limitation here is the availability of animal models to track the molecules and cells in their natural environment in vivo. Owing to the integrity of the studied tissue and the high stability of transgene expression throughout life, transgenic mice encoding fluorescent proteins and biosensors represent unique tools for in vivo studies in norm and pathology. We review the strategies for targeting probe expression in specific tissues, cell subtypes, or cellular compartments. We describe the application of transgenic mice expressing fluorescent proteins for tracking protein expression patterns, apoptotic events, tissue differentiation and regeneration, neurogenesis, tumorigenesis, and cell fate mapping. We overview the possibilities of functional imaging of secondary messengers, neurotransmitters, and ion fluxes. Finally, we provide the rationale and perspectives for the use of transgenic imaging probes in translational research and drug discovery.

Sensing Intra‐ and Extra‐Cellular Ca2+ in the Islet of Langerhans

Calcium ions (Ca2+) take part in intra‐ and inter‐cellular signaling to mediate cellular functions. Sensing this ubiquitous messenger is instrumental in disentangling the specific functions of cellular sub‐compartments and/or intercellular communications. In this review, the authors first describe intra‐ and inter‐cellular Ca2+ signaling in relation to insulin secretion from the pancreatic islets, and then outline the development of diverse sensors, for example, chemically synthesized indicators, genetically encoded proteins, and ion‐selective microelectrodes, for intra‐ and extra‐cellular sensing of Ca2+. Particular emphasis is placed on emerging approaches in this field, such as low‐affinity Ca2+ indicators and unique Ca2+‐responsive composite materials. The authors conclude by remarking on the challenges and opportunities for further developments in this field, which may facilitate a more comprehensive understanding of Ca2+ signaling within and outside the islets, and its relevance in health and disease.

Bioluminescent Sensors for Ca++ Flux Imaging and the Introduction of a New Intensity-Based Ca++ Sensor

Sensitive detection of biological events is a goal for the design and characterization of sensors that can be used in vitro and in vivo. One important second messenger is Ca⁺⁺ which has been a focus of using genetically encoded Ca⁺⁺ indicators (GECIs) within living cells or intact organisms in vivo. An ideal GECI would exhibit high signal intensity, excellent signal-to-noise ratio (SNR), rapid kinetics, a large dynamic range within relevant physiological conditions, and red-shifted emission. Most available GECIs are based on fluorescence, but bioluminescent GECIs have potential advantages in terms of avoiding tissue autofluorescence, phototoxicity, photobleaching, and spectral overlap, as well as enhancing SNR. Here, we summarize current progress in the development of bioluminescent GECIs and introduce a new and previously unpublished biosensor. Because these biosensors require a substrate, we also describe the pros and cons of various substrates used with these sensors. The novel GECI that is introduced here is called CalBiT, and it is a Ca⁺⁺ indicator based on the functional complementation of NanoBiT which shows a high dynamic change in response to Ca⁺⁺ fluxes. Here, we use CalBiT for the detection of Ca⁺⁺ fluctuations in cultured cells, including its ability for real-time imaging in living cells.

Imaging of Endoplasmic Reticulum Ca2+ in the Intact Pituitary Gland of Transgenic Mice Expressing a Low Affinity Ca2+ Indicator

The adenohypophysis contains five secretory cell types (somatotrophs, lactotrophs, thyrotrophs, corticotrophs, and gonadotrophs), each secreting a different hormone, and controlled by different hypothalamic releasing hormones (HRHs). Exocytic secretion is regulated by cytosolic Ca²⁺ signals ([Ca²⁺]_C), which can be generated either by Ca²⁺ entry through the plasma membrane and/or by Ca²⁺ release from the endoplasmic reticulum (ER). In addition, Ca²⁺ entry signals can eventually be amplified by ER release via calcium-induced calcium release (CICR). We have investigated the contribution of ER Ca²⁺ release to the action of physiological agonists in pituitary gland. Changes of [Ca²⁺] in the ER ([Ca²⁺]_ER) were measured with the genetically encoded low-affinity Ca²⁺ sensor GAP3 targeted to the ER. We used a transgenic mouse strain that expressed erGAP3 driven by a ubiquitous promoter. Virtually all the pituitary cells were positive for the sensor. In order to mimick the physiological environment, intact pituitary glands or acute slices from the transgenic mouse were used to image [Ca²⁺]_ER. [Ca²⁺]_C was measured simultaneously with Rhod-2. Luteinizing hormone-releasing hormone (LHRH) or thyrotropin releasing hormone (TRH), two agonists known to elicit intracellular Ca²⁺ mobilization, provoked robust decreases of [Ca²⁺]_ER and concomitant rises of [Ca²⁺]_C. A smaller fraction of cells responded to thyrotropin releasing hormone (TRH). By contrast, depolarization with high K⁺ triggered a rise of [Ca²⁺]_C without a decrease of [Ca²⁺]_ER, indicating that the calcium-induced calcium-release (CICR) via ryanodine receptor amplification mechanism is not present in these cells. Our results show the potential of transgenic ER Ca²⁺ indicators as novel tools to explore intraorganellar Ca²⁺ dynamics in pituitary gland in situ.

The contribution of organelles to plant intracellular Calcium signalling

Calcium (Ca2+) is among the most important intracellular messengers in living organisms. Understanding of the players and dynamics of Ca2+ signalling pathways in plants may help to unravel the molecular basis of their exceptional flexibility to respond and to adapt to different stimuli. In the present review we focus on new tools that have recently revolutionized our view of organellar Ca2+ signalling as well as on the current knowledge regarding the pathways mediating Ca2+ fluxes across intracellular membranes. The contribution of organelles and cellular subcompartments to the orchestrated response via Ca2+ signalling within a cell is also discussed, underlining the fact that one of the greatest challenges in the field is the elucidation of how influx and efflux Ca2+ transporters/channels are regulated in a concerted manner to translate specific information into a Ca2+ signature.

Using aequorin probes to measure Ca2+ in intracellular organelles

Aequorins are excellent tools for measuring intra-organellar Ca²⁺ and assessing its role in physiological and pathological functions. Here we review targeting strategies to express aequorins in various organelles. We address critical topics such as probe affinity tuning as well as normalization and calibration of the signal. We also focus on bioluminescent Ca²⁺ imaging in nucleus or mitochondria of living cells. Finally, recent advances with a new chimeric GFP-aequorin protein (GAP), which can be used either as luminescent or fluorescent Ca²⁺ probe, are presented. GAP is robustly expressed in transgenic flies and mice, where it has proven to be a suitable Ca²⁺ indicator for monitoring physiological Ca²⁺ signaling ex vivo and in vivo.

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.

Characterization of the ER-Targeted Low Affinity Ca(2+) Probe D4ER

Calcium ion (Ca²⁺) is a ubiquitous intracellular messenger and changes in its concentration impact on nearly every aspect of cell life. Endoplasmic reticulum (ER) represents the major intracellular Ca²⁺ store and the free Ca²⁺ concentration ([Ca²⁺]) within its lumen ([Ca²⁺]_ER) can reach levels higher than 1 mM. Several genetically-encoded ER-targeted Ca²⁺ sensors have been developed over the last years. However, most of them are non-ratiometric and, thus, their signal is difficult to calibrate in live cells and is affected by shifts in the focal plane and artifactual movements of the sample. On the other hand, existing ratiometric Ca²⁺ probes are plagued by different drawbacks, such as a double dissociation constant (K_d) for Ca²⁺, low dynamic range, and an affinity for the cation that is too high for the levels of [Ca²⁺] in the ER lumen. Here, we report the characterization of a recently generated ER-targeted, Förster resonance energy transfer (FRET)-based, Cameleon probe, named D4ER, characterized by suitable Ca²⁺ affinity and dynamic range for monitoring [Ca²⁺] variations within the ER. As an example, resting [Ca²⁺]_ER have been evaluated in a known paradigm of altered ER Ca²⁺ homeostasis, i.e., in cells expressing a mutated form of the familial Alzheimer’s Disease-linked protein Presenilin 2 (PS2). The lower Ca²⁺ affinity of the D4ER probe, compared to that of the previously generated D1ER, allowed the detection of a conspicuous, more clear-cut, reduction in ER Ca²⁺ content in cells expressing mutated PS2, compared to controls.

Genetically Encoded Fluorescent Indicators for Organellar Calcium Imaging

Optical Ca(2+) indicators are powerful tools for investigating intracellular Ca(2+) signals in living cells. Although a variety of Ca(2+) indicators have been developed, deciphering the physiological functions and spatiotemporal dynamics of Ca(2+) in intracellular organelles remains challenging. Genetically encoded Ca(2+) indicators (GECIs) using fluorescent proteins are promising tools for organellar Ca(2+) imaging, and much effort has been devoted to their development. In this review, we first discuss the key points of organellar Ca(2+) imaging and summarize the requirements for optimal organellar Ca(2+) indicators. Then, we highlight some of the recent advances in the engineering of fluorescent GECIs targeted to specific organelles. Finally, we discuss the limitations of currently available GECIs and the requirements for advancing the research on intraorganellar Ca(2+) signaling.

The prion protein constitutively controls neuronal store-operated Ca(2+) entry through Fyn kinase

The prion protein (PrP^C) is a cell surface glycoprotein mainly expressed in neurons, whose misfolded isoforms generate the prion responsible for incurable neurodegenerative disorders. Whereas PrP^C involvement in prion propagation is well established, PrP^C physiological function is still enigmatic despite suggestions that it could act in cell signal transduction by modulating phosphorylation cascades and Ca²⁺ homeostasis. Because PrP^C binds neurotoxic protein aggregates with high-affinity, it has also been proposed that PrP^C acts as receptor for amyloid-β (Aβ) oligomers associated with Alzheimer’s disease (AD), and that PrP^C-Aβ binding mediates AD-related synaptic dysfunctions following activation of the tyrosine kinase Fyn. Here, use of gene-encoded Ca²⁺ probes targeting different cell domains in primary cerebellar granule neurons (CGN) expressing, or not, PrP^C, allowed us to investigate whether PrP^C regulates store-operated Ca²⁺ entry (SOCE) and the implication of Fyn in this control. Our findings show that PrP^C attenuates SOCE, and Ca²⁺ accumulation in the cytosol and mitochondria, by constitutively restraining Fyn activation and tyrosine phosphorylation of STIM1, a key molecular component of SOCE. This data establishes the existence of a PrP^C-Fyn-SOCE triad in neurons. We also demonstrate that treating cerebellar granule and cortical neurons with soluble Aβ_(1–42) oligomers abrogates the control of PrP^C over Fyn and SOCE, suggesting a PrP^C-dependent mechanizm for Aβ-induced neuronal Ca²⁺ dyshomeostasis.

Generation of Red-Shifted Cameleons for Imaging Ca²⁺ Dynamics of the Endoplasmic Reticulum

Cameleons are sophisticated genetically encoded fluorescent probes that allow quantifying cellular Ca2+ signals. The probes are based on Förster resonance energy transfer (FRET) between terminally located fluorescent proteins (FPs), which move together upon binding of Ca2+ to the central calmodulin myosin light chain kinase M13 domain. Most of the available cameleons consist of cyan and yellow FPs (CFP and YFP) as the FRET pair. However, red-shifted versions with green and orange or red FPs (GFP, OFP, RFP) have some advantages such as less phototoxicity and minimal spectral overlay with autofluorescence of cells and fura-2, a prominent chemical Ca2+ indicator. While GFP/OFP- or GFP/RFP-based cameleons have been successfully used to study cytosolic and mitochondrial Ca2+ signals, red-shifted cameleons to visualize Ca2+ dynamics of the endoplasmic reticulum (ER) have not been developed so far. In this study, we generated and tested several ER targeted red-shifted cameleons. Our results show that GFP/OFP-based cameleons due to miss-targeting and their high Ca2+ binding affinity are inappropriate to record ER Ca2+ signals. However, ER targeted GFP/RFP-based probes were suitable to sense ER Ca2+ in a reliable manner. With this study we increased the palette of cameleons for visualizing Ca2+ dynamics within the main intracellular Ca2+ store.

MXS-Chaining: A Highly Efficient Cloning Platform for Imaging and Flow Cytometry Approaches in Mammalian Systems

The continuous improvement of imaging technologies has driven the development of sophisticated reporters to monitor biological processes. Such constructs should ideally be assembled in a flexible enough way to allow for their optimization. Here we describe a highly reliable cloning method to efficiently assemble constructs for imaging or flow cytometry applications in mammalian cell culture systems. We bioinformatically identified a list of restriction enzymes whose sites are rarely found in human and mouse cDNA libraries. From the best candidates, we chose an enzyme combination (MluI, XhoI and SalI: MXS) that enables iterative chaining of individual building blocks. The ligation scar resulting from the compatible XhoI- and SalI-sticky ends can be translated and hence enables easy in-frame cloning of coding sequences. The robustness of the MXS-chaining approach was validated by assembling constructs up to 20 kb long and comprising up to 34 individual building blocks. By assessing the success rate of 400 ligation reactions, we determined cloning efficiency to be 90% on average. Large polycistronic constructs for single-cell imaging or flow cytometry applications were generated to demonstrate the versatility of the MXS-chaining approach. We devised several constructs that fluorescently label subcellular structures, an adapted version of FUCCI (fluorescent, ubiquitination-based cell cycle indicator) optimized to visualize cell cycle progression in mouse embryonic stem cells and an array of artificial promoters enabling dosage of doxycyline-inducible transgene expression. We made publicly available through the Addgene repository a comprehensive set of MXS-building blocks comprising custom vectors, a set of fluorescent proteins, constitutive promoters, polyadenylation signals, selection cassettes and tools for inducible gene expression. Finally, detailed guidelines describe how to chain together prebuilt MXS-building blocks and how to generate new customized MXS-building blocks.

Methods to measure intracellular Ca(2+) fluxes with organelle-targeted aequorin-based probes

The photoprotein aequorin generates blue light upon binding of Ca(2+) ions. Together with its very low Ca(2+)-buffering capacity and the possibility to add specific targeting sequences, this property has rendered aequorin particularly suitable to monitor Ca(2+) concentrations in specific subcellular compartments. Recently, a new generation of genetically encoded Ca(2+) probes has been developed by fusing Ca(2+)-responsive elements with the green fluorescent protein (GFP). Aequorin has also been employed to this aim, resulting in an aequorin-GFP chimera with the Ca(2+) sensitivity of aequorin and the fluorescent properties of GFP. This setup has actually solved the major limitation of aequorin, for example, its poor ability to emit light, which rendered it inappropriate for the monitoring of Ca(2+) waves at the single-cell level by imaging. In spite of the numerous genetically encoded Ca(2+) indicators that are currently available, aequorin-based probes remain the method of election when an accurate quantification of Ca(2+) levels is required. Here, we describe currently available aequorin variants and their use for monitoring Ca(2+) waves in specific subcellular compartments. Among various applications, this method is relevant for the study of the alterations of Ca(2+) homeostasis that accompany oncogenesis, tumor progression, and response to therapy.

Aequorin as Intracellular Ca2+ Indicator Incorporated in Follicular Lymphoma Cells by Hypoosmotic Shock Treatment

Natural proteins can be used in measuring intracellular Ca(2+) concentration. As one of the Ca(2+)- regulated photoproteins, aequorin has several advantages in comparison to widely used Ca(2+) fluorescence indicators (e.g., fura-2, indo-1 and fluo-3), including high dynamic range and resistance to motion artefacts. However, incorporation of aequorin into cells remains a challenge. Hypoosmotic shock treatment was optimized and used as a method for loading aequorin into the cytoplasm of follicular lymphoma cells. Measurement of aequorin luminescence in the cells was performed using a luminometer with a sensitive photomultiplier tube and the luminescence intensity was recalculated into intracellular [Ca(2+)]. The value of (0.85 ± 0.52)·10-6 M was found. We show that the optimized method of incorporation was effective for loading aequorin into follicular lymphoma cells in vitro. The cell viability remains high immediately after the procedure. This method can also be used for measuring intracellular Ca(2+) concentration in other types of non-adherent cells.

Hydrophobic handoff for direct delivery of peroxisome tail-anchored proteins

Tail-anchored (TA) proteins are inserted into membranes post-translationally through a C-terminal transmembrane domain (TMD). The PEX19 protein binds peroxisome TA proteins in the cytoplasm and delivers them to the membrane through the PEX3 receptor protein. An amphipathic segment in PEX19 promotes docking on PEX3. However, how this leads to substrate insertion is unknown. Here we reconstitute peroxisome TA protein biogenesis into two sequential steps of substrate TMD engagement and membrane insertion. We identify a series of previously uncharacterized amphipathic segments in PEX19 and identify one whose hydrophobicity is required for membrane insertion, but not TMD chaperone activity or PEX3 binding. A membrane-proximal hydrophobic surface of PEX3 promotes an unconventional form of membrane intercalation, and is also required for TMD insertion. Together, these data support a mechanism in which hydrophobic moieties in the TMD chaperone and its membrane-associated receptor act in a concerted manner to prompt TMD release and membrane insertion.

Red fluorescent genetically encoded Ca2+ indicators for use in mitochondria and endoplasmic reticulum

Ca2+ is a key intermediary in a variety of signalling pathways and undergoes dynamic changes in its cytoplasmic concentration due to release from stores within the endoplasmic reticulum (ER) and influx from the extracellular environment. In addition to regulating cytoplasmic Ca2+ signals, these responses also affect the concentration of Ca2+ within the ER and mitochondria. Single fluorescent protein-based Ca2+ indicators, such as the GCaMP series based on GFP, are powerful tools for imaging changes in the concentration of Ca2+ associated with intracellular signalling pathways. Most GCaMP-type indicators have dissociation constants (Kd) for Ca2+ in the high nanomolar to low micromolar range and are therefore optimal for measuring cytoplasmic [Ca2+], but poorly suited for use in mitochondria and ER where [Ca2+] can reach concentrations of several hundred micromolar. We now report GCaMP-type low-affinity red fluorescent genetically encoded Ca2+ indicators for optical imaging (LAR-GECO), engineered to have Kd values of 24 μM (LAR-GECO1) and 12 μM (LAR-GECO1.2). We demonstrate that these indicators can be used to image mitochondrial and ER Ca2+ dynamics in several cell types. In addition, we perform two-colour imaging of intracellular Ca2+ dynamics in cells expressing both cytoplasmic GCaMP and ER-targeted LAR-GECO1. The development of these low-affinity intensiometric red fluorescent Ca2+ indicators enables monitoring of ER and mitochondrial Ca2+ in combination with GFP-based reporters.

GAP, an aequorin-based fluorescent indicator for imaging Ca2+ in organelles

Genetically encoded calcium indicators allow monitoring subcellular Ca(2+) signals inside organelles. Most genetically encoded calcium indicators are fusions of endogenous calcium-binding proteins whose functionality in vivo may be perturbed by competition with cellular partners. We describe here a novel family of fluorescent Ca(2+) sensors based on the fusion of two Aequorea victoria proteins, GFP and apo-aequorin (GAP). GAP exhibited a unique combination of features: dual-excitation ratiometric imaging, high dynamic range, good signal-to-noise ratio, insensitivity to pH and Mg(2+), tunable Ca(2+) affinity, uncomplicated calibration, and targetability to five distinct organelles. Moreover, transgenic mice for endoplasmic reticulum-targeted GAP exhibited a robust long-term expression that correlated well with its reproducible performance in various neural tissues. This biosensor fills a gap in the actual repertoire of Ca(2+) indicators for organelles and becomes a valuable tool for in vivo Ca(2+) imaging applications.

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.

Organellar calcium signalling mechanisms in Drosophilaepithelial function

Calcium signalling and calcium homeostasis are essential for life. Studies of calcium signalling thus constitute a major proportion of research in the life sciences, although the majority of these studies are based in cell lines or isolated cells. Epithelial cells and tissues are essential in the regulation of critical physiological processes, including fluid transport; and so the modulation of such processes in vivo by cell-specific calcium signalling is thus of interest. In this review, we describe the approaches to measuring intracellular calcium in the genetically tractable fluid-transporting tissue, the Drosophila Malpighian tubule by targeting cell-specific protein-based calcium reporters to defined regions,cells and intracellular compartments of the intact Malpighian tubule. We also discuss recent findings on the roles of plasma membrane and intracellular calcium channels; and on organellar stores – including mitochondria,Golgi and peroxisomes – in Malpighian tubule function.

Luminescence of imidazo[1,2-a]pyrazin-3(7H)-one compounds

In this review I will discuss chemical principles of the luminescence of imidazo[1,2-a]pyrazin-3(7H)-one compounds described to date. The review is composed of two main parts, the first dealing with the bioluminescence of coelenterate luciferin "coelenterazine" and Cypridina luciferin in marine organisms and the second with the chemiluminescence of these luciferins and their analogues. In the second section, possible applications of chemiluminescence and enhanced chemiluminescence in the area of bioassay are also discussed.

Calcium in plant defence-signalling pathways

In plant cells, the calcium ion is a ubiquitous intracellular second messenger involved in numerous signalling pathways. Variations in the cytosolic concentration of Ca2+ ([Ca2+]cyt) couple a large array of signals and responses. Here we concentrate on calcium signalling in plant defence responses, particularly on the generation of the calcium signal and downstream calcium-dependent events participating in the establishment of defence responses with special reference to calcium-binding proteins.

Expression and optical properties of green fluorescent protein expressed in different cellular environments

This study has investigated the expression of green fluorescent protein (GFP) variants in the cytosol and the endoplasmic reticulum (ER) of HeLa cells and evaluated the effects of the different cellular environments on the fluorescence properties of these GFP variants. Several GFP variants have been constructed by adding different N- or C-terminal signal sequences. These proteins were expressed and folded in distinct cellular compartments in HeLa cells. The localization of these GFP variants targeted to the endoplasmic recticulum was confirmed by the co-localization of DsRed2-ER as assessed by confocal microscopy. The addition of signal peptides targeting GFP variants to the ER or cytosol did not appear to alter the optical spectra of these GFP variants. However, the fluorescence intensity of these GFP variants in the ER was significantly less than that in the cytosol. Thus, the results clearly suggest that the cellular environment affects the formation and/or maturation of green fluorescence protein in vivo. These findings will be helpful in the future development and application of GFP technology aimed at investigating cellular functions performed in the ER and the cytosol.

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

Calcium measurement in living filamentous fungi expressing codon-optimized aequorin

Calcium signalling is little understood in filamentous fungi largely because easy and routine methods for calcium measurement in living hyphae have previously been unavailable. We have developed the recombinant aequorin method for this purpose. High levels of aequorin expression were obtained in Neurospora crassa, Aspergillus niger and Aspergillus awamori by codon optimization of the aequorin gene. Three external stimuli (mechanical perturbation, hypo-osmotic shock and high external calcium) were found transiently to increase [Ca(2+)](c). Each of the calcium signatures associated with these physico-chemical treatments was unique, suggesting the involvement of three distinct calcium-mediated signal transduction pathways. The fungal calcium channel blocker KP4 inhibited the [Ca(2+)](c) responses to hypo-osmotic shock and high external calcium, but not to mechanical perturbation. The divalent cation chelator BAPTA inhibited [Ca(2+)](c) responses to mechanical perturbation and hypo-osmotic shock. The calcium agonists A23187 and cyclopiazonic acid increased [Ca(2+)](c) levels.

Characterization of Coelenterazine Analogs for Measurements of Renilla Luciferase Activity in Live Cells and Living Animals

In vivo imaging of bioluminescent reporters relies on expression of light-emitting enzymes, luciferases, and delivery of chemical substrates to expressing cells. Coelenterazine (CLZN) is the substrate for a group of bioluminescent enzymes obtained from marine organisms. At present, there are more than 10 commercially available CLZN analogs. To determine which analog is most suitable for activity measurements in live cells and living animals, we characterized 10 CLZN analogs using Renilla luciferase (Rluc) as the reporter enzyme. For each analog, we monitored enzyme activity, auto-oxidation, and efficiency of cellular uptake. All CLZN analogs tested showed higher auto-oxidation signals in serum than was observed in phosphate buffer or medium, mainly as a result of auto-oxidation by binding to albumin. CLZN-f, -h, and -e analogs showed 4- to 8-fold greater Rluc activity, relative to CLZN-native, in cells expressing the enzyme from a stable integrant. In studies using living mice expressing Rluc in hepatocytes, administration of CLZN-e and -native produced the highest signal. Furthermore, distinct temporal differences in signal for each analog were revealed following intravenous or intraperitoneal delivery. We conclude that the CLZN analogs that are presently available vary with respect to hRluc utilization in culture and in vivo, and that the effective use of CLZN-utilizing enzymes in living animals depends on the selection of an appropriate substrate.

Measuring [Ca2+] in the endoplasmic reticulum with aequorin

The photoprotein aequorin was the first probe used to measure specifically the [Ca(2+)] inside the lumen of the endoplasmic reticulum ([Ca(2+)](ER)) of intact cells and it provides values for the steady-state [Ca(2+)](ER), around 500 microM, that closely match those obtained now by other procedures. Aequorin-based methods to measure [Ca(2+)](ER) offer several advantages: (i) targeting of the probe is extremely precise; (ii) the use of low Ca(2+)-affinity aequorin allows covering a large dynamic range of [Ca(2+)], from 10(-5) to 10(-3)M; (iii) aequorin is nearly insensitive to changes in Mg(2+) or pH, has a high signal-to-noise ratio and calibration of the results in [Ca(2+)] is made straightforward using a simple algorithm; and (iv) the equipment required for luminescence measurements in cell populations is simple and low-cost. On the negative side, this technique has also some disadvantages: (i) the relatively low amount of emitted light makes difficult performing single-cell imaging studies; (ii) reconstitution of aequorin with coelenterazine requires previous complete depletion of Ca(2+) of the ER for 1-2h, a maneuver that may result in deleterious effects in some cells; (iii) because of the high rate of aequorin consumption at steady-state [Ca(2+)](ER), only relatively brief experiments can be performed; and (iv) expression of ER-targeted aequorin requires previous transfection or infection to introduce the appropriate DNA construct, or alternatively the use of stable cell clones. Choosing aequorin or other techniques to measure [Ca(2+)](ER) will depend of the correct balance between these properties in a particular problem.

Reducing the environmental sensitivity of yellow fluorescent protein. Mechanism and applications

Yellow mutants of the green fluorescent protein (YFP) are crucial constituents of genetically encoded indicators of signal transduction and fusions to monitor protein-protein interactions. However, previous YFPs show excessive pH sensitivity, chloride interference, poor photostability, or poor expression at 37 degrees C. Protein evolution in Escherichia coli has produced a new YFP named Citrine, in which the mutation Q69M confers a much lower pK(a) (5.7) than for previous YFPs, indifference to chloride, twice the photostability of previous YFPs, and much better expression at 37 degrees C and in organelles. The halide resistance is explained by a 2.2-A x-ray crystal structure of Citrine, showing that the methionine side chain fills what was once a large halide-binding cavity adjacent to the chromophore. Insertion of calmodulin within Citrine or fusion of cyan fluorescent protein, calmodulin, a calmodulin-binding peptide and Citrine has generated improved calcium indicators. These chimeras can be targeted to multiple cellular locations and have permitted the first single-cell imaging of free [Ca(2+)] in the Golgi. Citrine is superior to all previous YFPs except when pH or halide sensitivity is desired and is particularly advantageous within genetically encoded fluorescent indicators of physiological signals.

Protection against nitrofurantoin-induced oxidative stress by coelenterazine analogues and their oxidation products in rat hepatocytes

Coelenterazine (3,7-dihydro-2-(p-hydroxybenzyl)-6-(p-hydroxyphenyl)-8-benzylimidazolo[1,2-a]pyrazin-3- one) is a substrate for the bioluminescence reaction in many marine animals. Recent work showed that CLZn, its synthetic analogue CLZm, and their common oxidation product coelenteramine (CLM) have strong antioxidative properties in acellular lipid peroxidation systems as well as in rat hepatocytes subjected to tert-butyl hydroperoxide (t-BHP). Here, we analyzed the ability of CLZm and several imidazolopyrazinone (IMPZs) analogues to protect primary cultures of rat hepatocytes against a nitrofurantoin (NF)-induced oxidative stress. Comparison of protection capabilities with reference antioxidants yielded the following ranking: CLZm >>> BHT >Trolox C((R)) > probucol > alpha-tocopherol. The comparison of CLZm with analogues lacking the phenol group in R(1) revealed no differences although the presence of this phenol conferred superior protection against t-BHP. CLM, as well as its methoxylated analogue mCLM which lacks chain-breaking properties, were equally potent in preventing cellular damage caused by NF. mCLM and alpha-naphthoflavone, an inhibitor of cytochrome P450 (CYP450) IAI, similarly protected cells against NF-induced mortality and also equally inhibited EROD activity in methylcholanthrene-induced hepatocytes. The inhibition of EROD by CLZm and CLM was less pronounced. We suggest that the extent of protection conferred by IMPZs against NF-toxicity reflects both the occurrence of antioxidative properties detoxifying ROS produced within cells and inhibitory actions on CYP450 isoforms involved in the bioreduction of NF.

Mag-indo1 affinity for Ca(2+), compartmentalization and binding to proteins: the challenge of measuring Mg(2+) concentrations in living cells

A physicochemical study of the Mag-indo1 binding to Ca(2+) in solution showed that: (i) the characteristic fluorescence spectra of Ca(2+)-bound and Mg(2+)-bound Mag-indo1 are identical; (ii) two successive equilibria occur for increasing Ca(2+) concentrations; and (iii) the value of the dissociation constant of the first one, as determined by using a probe dilution protocol, amounts to 780 nM. In order to investigate the fluorescence level of Mag-indo1 trapped in cell organelles, fluorescence spectra of Mag-indo1-loaded fibroblasts were recorded before and after a digitonin permeabilization. Their resolution into cation-bound, protein-bound, and free Mag-indo1 characteristic spectra allowed measurement of the fluorescence intensities of these species. The intensities emitted from whole cells were compared to those emitted from organelles (assumed to be endoplasmic reticulum according to a DiOC(6) loading). The cation-bound Mag-indo1 fluorescence resulted partially (20 to 50%) from the cytosol for 30% of the cells, and totally from compartments for 70% of the cells. We found a concentration value of 500 nM for compartmentalized Ca(2+) and concluded that the Mag-indo1 binding to Ca(2+) is likely to affect drastically the Mg(2+) concentration measurements in cells. Moreover, we showed that the amount variation of protein-bound Mag-indo1 also affects Mg(2+) measurements when using the two-wavelength ratio method.

Antioxidative properties of natural coelenterazine and synthetic methyl coelenterazine in rat hepatocytes subjected to tert-butyl hydroperoxide-induced oxidative stress

Coelenterazine (CLZn; 3, 7-dihydro-2-(p-hydroxybenzyl)-6-(p-hydroxyphenyl)-8-benzylimidazolo++ +[1 ,2-a]pyrazin-3-one), the substrate for bioluminescence reactions in many marine animals, is endowed with high antioxidant properties. This work investigated the antioxidative properties of CLZn in primary cultures of rat hepatocytes subjected to the oxidant tert-butyl hydroperoxide (t-BHP). Micromolar concentrations of CLZn increased survival and decreased lipid peroxidation in rat hepatocytes subjected for 6 hr to 2.5 x 10(-4) M t-BHP. However, the extent of protection was limited by a strong toxicity of CLZn (IC(50) = 6.9 x 10(-5) M). The presence of t-BHP increased the cellular toxicity of CLZn. Methyl coelenterazine (CLZm, 3, 7-dihydro-2-methyl-6-(p-hydroxyphenyl)-8 benzylimidazolo[1, 2-a]pyrazin-3-one), a synthetic analogue of CLZn, demonstrated excellent antioxidant properties, even at very low (3 x 10(-6) M) concentrations and was not toxic throughout most of its effective concentration range. CLZm proved far more effective than reference antioxidants such as Trolox C(R), alpha-tocopherol, BHT, and probucol. The assay of thiobarbituric reactive substances (TBARS) associated with cells and in the culture medium indicated that 10(-5) M CLZm provided a total protection against t-BHP-induced lipid peroxidation. This coelenterazine analogue could be used as a model compound for investigating the action mechanism of imidazolopyrazinones in mammalian hepatocytes.

Cloning and expression of the bioluminescent photoprotein pholasin from the bivalve mollusc Pholas dactylus

Pholasin is the photoprotein responsible for luminescence in the bivalve Pholas dactylus and consists of a luciferin tightly bound to a glycosylated protein. It is a sensitive indicator of reactive oxygen species. A full-length clone encoding apopholasin was isolated from a P. dactylus light organ cDNA library. The unprocessed apoprotein contained 225 amino acids, starting with a signal peptide of 20 amino acids, 3 predicted N-linked glycosylation sites, 1 O-linked site, no histidines, and 7 cysteines. The recombinant apoprotein was expressed in cell extracts and insect cells. The size of the apoprotein expressed in cell extracts and the cytosol of insect cells was 26 kDa but that of the fully processed protein was 34 kDa, as was native pholasin. Both the processed and unprocessed recombinant apoproteins were recognized by a polyclonal antibody raised against native pholasin. Acid methanol extracts from Pholas added to recombinant apoprotein resulted in chemiluminescence triggered by sodium hypochlorite but not photoprotein formation. These results have important implications in understanding the molecular evolution of bioluminescence and will allow the development of recombinant pholasin as an intracellular indicator of reactive oxygen species.

Apoaequorin monitors degradation of endoplasmic reticulum (ER) proteins initiated by loss of ER Ca(2+)

Apoaequorin was targeted to the cytosol, nucleus, and endoplasmic reticulum of HeLa cells in order to determine the effect of Ca(2+) release from the ER on protein degradation. In resting cells apoaequorin had a rapid half-life (ca. 20-30 min) in the cytosol or nucleus, but was relatively stable for up to 24 h in the ER (t(1/2) > 24 h). However, release of Ca(2+) from the ER, initiated by the addition of inhibitors of the ER Ca(2+)/Mg(2+) ATPase such as 2 microM thapsigargin or 1 microM ionomycin, initiated rapid loss of apoaequorin in the ER, but had no detectable effect on apoaequorin turnover in the cytosol nor the nucleus. This loss of apoprotein was not the result of secretion into the external fluid, and could not be inhibited by inhibitors of protein degradation by proteosomes. Proteolysis of apoaequorin in cell extracts (t(1/2) < 20 min) was completely inhibited in the presence of 1 mM Ca(2+), and this effect was independent of the ER retention signal KDEL at the C-terminus. Proteolysis was unaffected by the presence of selected serine protease inhibitors, or 10 microM Zn(2+), a known caspase-3 inhibitor. The results show that apoaequorin can monitor proteolysis of ER proteins activated by loss of ER Ca(2+). Several Ca(2+)-binding proteins exist in the ER, acting as the Ca(2+) store and chaperones. Our results have important implications both for the role of ER Ca(2+) in cell activation and stress and when using aequorin for monitoring free ER Ca(2+) over long time periods.

Measurement of intracellular calcium

To a certain extent, all cellular, physiological, and pathological phenomena that occur in cells are accompanied by ionic changes. The development of techniques allowing the measurement of such ion activities has contributed substantially to our understanding of normal and abnormal cellular function. Digital video microscopy, confocal laser scanning microscopy, and more recently multiphoton microscopy have allowed the precise spatial analysis of intracellular ion activity at the subcellular level in addition to measurement of its concentration. It is well known that Ca2+ regulates numerous physiological cellular phenomena as a second messenger as well as triggering pathological events such as cell injury and death. A number of methods have been developed to measure intracellular Ca2+. In this review, we summarize the advantages and pitfalls of a variety of Ca2+ indicators used in both optical and nonoptical techniques employed for measuring intracellular Ca2+ concentration.

Minimal model of arterial chaos generated by coupled intracellular and membrane Ca2+ oscillators

We have developed a mathematical model of arterial vasomotion in which irregular rhythmic activity is generated by the nonlinear interaction of intracellular and membrane oscillators that depend on cyclic release of Ca2+ from internal stores and cyclic influx of extracellular Ca2+, respectively. Four key control variables were selected on the basis of the pharmacological characteristics of histamine-induced vasomotion in rabbit ear arteries: Ca2+ concentration in the cytosol, Ca2+ concentration in ryanodine-sensitive stores, cell membrane potential, and the open state probability of Ca2+-activated K+ channels. Although not represented by independent dynamic variables, the model also incorporates Na+/Ca2+ exchange, the Na+-K+-ATPase, Cl- fluxes, and Ca2+ efflux via the extrusion ATPase. Simulations reproduce a wide spectrum of experimental observations, including 1) the effects of interventions that modulate the functionality of Ca2+ stores and membrane ion channels, 2) paradoxes such as the apparently unpredictable dual action of Ca2+ antagonists and low extracellular Na+ concentration, which can abolish vasomotion or promote the appearance of large-amplitude oscillations, and 3) period-doubling, quasiperiodic, and intermittent routes to chaos. Nonlinearity is essential to explain these diverse patterns of experimental vascular response.

Secretagogues modulate the calcium concentration in the endoplasmic reticulum of insulin-secreting cells. Studies in aequorin-expressing intact and permeabilized ins-1 cells

The precise regulation of the Ca2+ concentration in the endoplasmic reticulum ([Ca2+]er) is important for protein processing and signal transduction. In the pancreatic beta-cell, dysregulation of [Ca2+]er may cause impaired insulin secretion. The Ca2+-sensitive photoprotein aequorin mutated to lower its Ca2+ affinity was stably expressed in the endoplasmic reticulum (ER) of rat insulinoma INS-1 cells. The steady state [Ca2+]er was 267 +/- 9 microM. Both the Ca2+-ATPase inhibitor cyclopiazonic acid and 4-chloro-m-cresol, an activator of ryanodine receptors, caused an almost complete emptying of ER Ca2+. The inositol 1,4,5-trisphosphate generating agonists, carbachol, and ATP, reduced [Ca2+]er by 20-25%. Insulin secretagogues that raise cytosolic [Ca2+] by membrane depolarization increased [Ca2+]er in the potency order K+ >> glucose > leucine, paralleling their actions in the cytosolic compartment. Glucose, which augmented [Ca2+]er by about 25%, potentiated the Ca2+-mobilizing effect of carbachol, explaining the corresponding observation in cytosolic [Ca2+]. The filling of ER Ca2+ by glucose is not directly mediated by ATP production as shown by the continuous monitoring of cytosolic ATP in luciferase expressing cells. Both glucose and K+ increase [Ca2+]er, but only the former generated whereas the latter consumed ATP. Nonetheless, drastic lowering of cellular ATP with a mitochondrial uncoupler resulted in a marked decrease in [Ca2+]er, emphasizing the requirement for mitochondrially derived ATP above a critical threshold concentration. Using alpha-toxin permeabilized cells in the presence of ATP, glucose 6-phosphate did not change [Ca2+]er, invalidating the hypothesis that glucose acts through this metabolite. Therefore, insulin secretagogues that primarily stimulate Ca2+ influx, elevate [Ca2+]er to ensure beta-cell homeostasis.

Functional measurements of [Ca2+] in the endoplasmic reticulum using a herpes virus to deliver targeted aequorin

Changes in the free calcium concentration of the endoplasmic reticulum ([Ca2+]er) play a central role controlling cellular functions like contraction, secretion or neuronal signaling. We recently reported that recombinant aequorin targeted to the endoplasmic reticulum (ER) [Montero M., Brini M., Marsault R. et al. Monitoring dynamic changes in free Ca2+ concentration in the endoplasmic reticulum of intact cells. EMBO J 1995; 14: 5467-5475, Montero M., Barrero M.J., Alvarez J. [Ca2+] microdomains control agonist-induced Ca2+ release in intact cells. FASEB J 1997; 11: 881-886] can be used to monitor selectively [Ca2+]er in intact HeLa cells. Here we have used a herpes simplex virus type 1 (HSV-1) based system to deliver targeted aequorin into a number of different cell types including both postmitotic primary cells (anterior pituitary cells, chromaffin cells and cerebellar neurons) and cell lines (HeLa, NIH3T3, GH3 and PC12 cells). Functional studies showed that the steady state lumenal [Ca2+]er ranged from around 300 microM in granule cells to 800 microM in GH3 cells. InsP3-coupled receptor stimulation with agonists like histamine (in HeLa, NIH3T3 and chromaffin cells), UTP and bradykinin (in PC12 cells) or thyrotropin-releasing hormone (TRH, in GH3 cells) produced a very rapid decrease in lumenal [Ca2+]er. Caffeine caused a rapid Ca2+ depletion of the ER in chromaffin cells, but not in the other cell types. Depolarization by high K+ produced an immediate and reversible increase of [Ca2+]er in all the excitable cells (anterior pituitary, GH3, chromaffin cells and granule neurons). We conclude that delivery of recombinant aequorin to the ER using HSV amplicon provides the first direct quantitative and dynamic measurements of [Ca2+]er in several primary non-dividing cells.

Aequorea victoria bioluminescence moves into an exciting new era

Bioluminescence has revolutionized research into many cellular and molecular-biological processes, ranging from intracellular signalling to gene transcription. This article focuses on the chemistry and biotechnological exploitation of the two proteins involved in bioluminescence of the jellyfish Aequorea victoria--aequorin and green fluorescent protein. Engineered recombinant aequorin has led to a novel technological approach to monitoring calcium signals in organelles and subcellular domains. A new generation of intracellular calcium indicators has been produced in which engineered variants of green fluorescent protein are used to probe their ionic environment using intramolecular fluorescence-resonance-energy transfer.

Assembly of chimeric connexin-aequorin proteins into functional gap junction channels. Reporting intracellular and plasma membrane calcium environments

Chimeric proteins comprising connexins 26, 32, and 43 and aequorin, a chemiluminescent calcium indicator, were made by fusing the amino terminus of aequorin to the carboxyl terminus of connexins. The retention of function by the chimeric partners was investigated. Connexin 32-aequorin and connexin 43-aequorin retained chemiluminescent activity whereas that of connexin 26-aequorin was negligible. Immunofluorescent staining of COS-7 cells expressing the chimerae showed they were targeted to the plasma membrane. Gap junction intercellular channel formation by the chimerae alone and in combination with wild-type connexins was investigated. Stable HeLa cells expressing connexin 43-aequorin were functional, as demonstrated by Lucifer yellow transfer. Paris of Xenopus oocytes expressing connexin 43-aequorin were electrophysiologically coupled, but those expressing chimeric connexin 26 or 32 showed no detectable levels of coupling. The formation of heteromeric channels constructed of chimeric connexin 32 or connexin 43 and the respective wild-type connexins was inferred from the novel voltage gating properties of the junctional conductance. The results show that the preservation of function by each partner of the chimeric protein is dictated mainly by the nature of the connexin, especially the length of the cytoplasmic carboxyl-terminal domain. The aequorin partner of the connexin 43 chimera reported calcium levels in COS-7 cells in at least two different calcium environments.

The endoplasmic reticulum Ca2+ store: a view from the lumen

The endoplasmic reticulum (ER) and its specialized subcompartments such as the sarcoplasmic reticulum, is the main dynamic Ca2+ storage compartment of the cell. Key cellular functions are regulated, either directly or indirectly, by the free Ca2+ concentration in the ER. This article discusses the properties of Ca2+ storage in the ER and considers the functions that appear to be regulated by the Ca2+ stores within the ER, both in and around the ER and at a distance from it.

Selective transfer of calcium from an acidic compartment to the mitochondrion of Trypanosoma brucei. Measurements with targeted aequorins

Organelle compartments are used by cells as reservoirs of exchangeable Ca2+ and as Ca2+ buffers. The following study uses recombinant aequorins (CYT-AEQ and MT-AEQ) to measure the dynamics of Ca2+ flux between organelles in procyclic forms of the pathogenic protozoan, Trypanosoma brucei. Emphasis is placed on the exchange between an acidic Ca2+ reservoir and the mitochondrion. The mammalian mitochondrial targeting sequence was functional in trypanosomes as determined by immunoblots, immunolocalizations, and the observation that MT-AEQ was in a compartment whose Ca2+ uptake was inhibited 82% with carbonyl cyanide p-trifluoromethoxyphenylhydrazone and KCN. The resting level of free calcium ion concentration in the mitochondrion ([Ca2+]mit) was slightly higher than that in the cytoplasm ([Ca2+]cyt) (400 +/- 50 nM and 290 +/- 40 nM, respectively). Melittin (125 nM) disrupted Ca2+ homeostasis by inducing Ca2+ influx across the plasma membrane. [Ca2+]cyt became slightly elevated to 410 +/- 100 nM, whereas [Ca2+]mit was selectively increased approximately 12-fold, with a broad peak at 4.8 +/- 1.9 microM. At the peak, the mitochondrion contained approximately three times more free Ca2+ than the cytosol. However, mitochondrial retention of the Ca2+ was transient. Similar selective transport into the mitochondrion was observed when Ca2+ efflux from an acidic compartment was induced with monensin (2 microg/ml) in the presence of 5 mM EGTA. [Ca2+]cyt was transiently elevated to 400 +/- 50 nM, whereas [Ca2+]mit was elevated to 3.3+/-1.3 microM. When cells were treated sequentially with monensin (2 microg/ml) and then melittin (200 nM), mitochondrial Ca2+ transport was normal. However, [Ca2+]cyt became elevated to a level that was 1.4-fold higher than with melittin alone. Overall, these data demonstrate that the trypanosome mitochondrion is not a reservoir of exchangeable Ca2+ in the resting cell. However, Ca2+ is selectively channeled to the mitochondrion from the plasma membrane or acidic Ca2+ storage compartment. Additionally, the acidic compartment contributes to maintenance of Ca2+ homeostasis in response to melittin.

Ca2+ homeostasis in the endoplasmic reticulum: coexistence of high and low [Ca2+] subcompartments in intact HeLa cells

Two recombinant aequorin isoforms with different Ca2+ affinities, specifically targeted to the endoplasmic reticulum (ER), were used in parallel to investigate free Ca2+ homeostasis in the lumen of this organelle. Here we show that, although identically and homogeneously distributed in the ER system, as revealed by both immunocytochemical and functional evidence, the two aequorins measured apparently very different concentrations of divalent cations ([Ca2+]er or [Sr2+]er). Our data demonstrate that this contradiction is due to the heterogeneity of the [Ca2+] of the aequorin-enclosing endomembrane system. Because of the characteristics of the calibration procedure used to convert aequorin luminescence into Ca2+ concentration, the [Ca2+]er values obtained at steady state tend, in fact, to reflect not the average ER values, but those of one or more subcompartments with lower [Ca2+]. These subcompartments are not generated artefactually during the experiments, as revealed by the dynamic analysis of the ER structure in living cells carried out by means of an ER-targeted green fluorescent protein. When the problem of ER heterogeneity was taken into account (and when Sr2+ was used as a Ca2+ surrogate), the bulk of the organelle was shown to accumulate free [cation2+]er up to a steady state in the millimolar range. A theoretical model, based on the existence of multiple ER subcompartments of high and low [Ca2+], that closely mimics the experimental data obtained in HeLa cells during accumulation of either Ca2+ or Sr2+, is presented. Moreover, a few other key problems concerning the ER Ca2+ homeostasis have been addressed with the following conclusions: (a) the changes induced in the ER subcompartments by receptor generation of InsP3 vary depending on their initial [Ca2+]. In the bulk of the system there is a rapid release whereas in the small subcompartments with low [Ca2+] the cation is simultaneously accumulated; (b) stimulation of Ca2+ release by receptor-generated InsP3 is inhibited when the lumenal level is below a threshold, suggesting a regulation by [cation2+]er of the InsP3 receptor activity (such a phenomenon had already been reported, however, but only in subcellular fractions analyzed in vitro); and (c) the maintenance of a relatively constant level of cytosolic [Ca2+], observed when the cells are incubated in Ca2+-free medium, depends on the continuous release of the cation from the ER, with ensuing activation in the plasma membrane of the channels thereby regulated (capacitative influx).