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

Using Fluorescent GAP Indicators to Monitor ER Ca2

The endoplasmic reticulum (ER) is the main reservoir of Ca2+ of the cell. Accurate and quantitative measuring of Ca2+ dynamics within the lumen of the ER has been challenging. In the last decade a few genetically encoded Ca2+ indicators have been developed, including a family of fluorescent Ca2+ indicators, dubbed GFP-Aequorin Proteins (GAPs). They are based on the fusion of two jellyfish proteins, the green fluorescent protein (GFP) and the Ca2+-binding protein aequorin. GAP Ca2+ indicators exhibit a combination of several features: they are excitation ratiometric indicators, with reciprocal changes in the fluorescence excited at 405 and 470 nm, which is advantageous for imaging experiments; they exhibit a Hill coefficient of 1, which facilitates the calibration of the fluorescent signal into Ca2+ concentrations; they are insensible to variations in the Mg2+ concentrations or pH variations (in the 6.5-8.5 range); and, due to the lack of mammalian homologues, these proteins have a favorable expression in transgenic animals. A low Ca2+ affinity version of GAP, GAP3 (KD ≅ 489 µM), has been engineered to conform with the estimated [Ca2+] in the ER. GAP3 targeted to the lumen of the ER (erGAP3) can be utilized for imaging intraluminal Ca2+. The ratiometric measurements provide a quantitative method to assess accurate [Ca2+]ER, both dynamically and at rest. In addition, erGAP3 can be combined with synthetic cytosolic Ca2+ indicators to simultaneously monitor ER and cytosolic Ca2+. Here, we provide detailed methods to assess erGAP3 expression and to perform Ca2+ imaging, either restricted to the ER lumen, or simultaneously in the ER and the cytosol. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Detection of erGAP3 in the ER by immunofluorescence Basic Protocol 2: Monitoring ER Ca2+ Basic Protocol 3: Monitoring ER- and cytosolic-Ca2+ Support Protocol: Generation of a stable cell line expressing erGAP3.

Opticool: Cutting-edge transgenic optical tools

Only a few short decades have passed since the sequencing of GFP, yet the modern repertoire of transgenically encoded optical tools implies an exponential proliferation of ever improving constructions to interrogate the subcellular environment. A myriad of tags for labeling proteins, RNA, or DNA have arisen in the last few decades, facilitating unprecedented visualization of subcellular components and processes. Development of a broad array of modern genetically encoded sensors allows real-time, in vivo detection of molecule levels, pH, forces, enzyme activity, and other subcellular and extracellular phenomena in ever expanding contexts. Optogenetic, genetically encoded optically controlled manipulation systems have gained traction in the biological research community and facilitate single-cell, real-time modulation of protein function in vivo in ever broadening, novel applications. While this field continues to explosively expand, references are needed to assist scientists seeking to use and improve these transgenic devices in new and exciting ways to interrogate development and disease. In this review, we endeavor to highlight the state and trajectory of the field of in vivo transgenic optical tools.

In vitro and in vivo calibration of low affinity genetic Ca2+ indicators

Calcium is a universal intracellular messenger and proper Ca2+concentrations ([Ca2+]) both in the cytosol and in the lumen of cytoplasmic organelles are essential for cell functions. Ca2+ homeostasis is achieved by a delicate pump/leak balance both at the plasma membrane and at the endomembranes, and improper Ca2+ levels result in malfunction and disease. Selective intraorganellar Ca2+measurements are best achieved by using targeted genetically encoded Ca2+ indicators (GECIs) but to calibrate the luminal fluorescent signals into accurate [Ca2+] is challenging, especially in vivo, due to the difficulty to normalize and calibrate the fluorescent signal in various tissues or conditions. We report here a procedure to calibrate the ratiometric signal of GAP (GFP-Aequorin Protein) targeted to the endo-sarcoplasmic reticulum (ER/SR) into [Ca2+]ER/SR based on imaging of fluorescence after heating the tissue at 50-52 °C, since this value coincides with that obtained in the absence of Ca2+ (Rmin). Knowledge of the dynamic range (Rmax/Rmin) and the Ca2+-affinity (KD) of the indicator permits calculation of [Ca2+] by applying a simple algorithm. We have validated this procedure in vitro using several cell types (HeLa, HEK 293T and mouse astrocytes), as well as in vivo in Drosophila. Moreover, this methodology is applicable to other low Ca2+ affinity green and red GECIs.

TRPV1 Channels Are New Players in the Reticulum-Mitochondria Ca2+ Coupling in a Rat Cardiomyoblast Cell Line

The Ca2+ release in microdomains formed by intercompartmental contacts, such as mitochondria-associated endoplasmic reticulum membranes (MAMs), encodes a signal that contributes to Ca2+ homeostasis and cell fate control. However, the composition and function of MAMs remain to be fully defined. Here, we focused on the transient receptor potential vanilloid 1 (TRPV1), a Ca2+-permeable ion channel and a polymodal nociceptor. We found TRPV1 channels in the reticular membrane, including some at MAMs, in a rat cardiomyoblast cell line (SV40-transformed H9c2) by Western blotting, immunostaining, cell fractionation, and proximity ligation assay. We used chemical and genetic probes to perform Ca2+ imaging in four cellular compartments: the endoplasmic reticulum (ER), cytoplasm, mitochondrial matrix, and mitochondrial surface. Our results showed that the ER Ca2+ released through TRPV1 channels is detected at the mitochondrial outer membrane and transferred to the mitochondria. Finally, we observed that prolonged TRPV1 modulation for 30 min alters the intracellular Ca2+ equilibrium and influences the MAM structure or the hypoxia/reoxygenation-induced cell death. Thus, our study provides the first evidence that TRPV1 channels contribute to MAM Ca2+ exchanges.

Use of aequorin-based indicators for monitoring Ca2+ in acidic organelles

Over the last years, there is accumulating evidence that acidic organelles can accumulate and release Ca²⁺ upon cell activation. Hence, reliable recording of Ca²⁺ dynamics in these compartments is essential for understanding the physiopathological aspects of acidic organelles. Genetically encoded Ca²⁺ indicators (GECIs) are valuable tools to monitor Ca²⁺ in specific locations, although their use in acidic compartments is challenging due to the pH sensitivity of most available fluorescent GECIs. By contrast, bioluminescent GECIs have a combination of features (marginal pH sensitivity, low background, no phototoxicity, no photobleaching, high dynamic range and tunable affinity) that render them advantageous to achieve an enhanced signal-to-noise ratio in acidic compartments. This article reviews the use of bioluminescent aequorin-based GECIs targeted to acidic compartments. A need for more measurements in highly acidic compartments is identified.

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.

Shaped by leaky ER: Homeostatic Ca2+ fluxes

At any moment in time, cells coordinate and balance their calcium ion (Ca²⁺) fluxes. The term ‘Ca²⁺ homeostasis’ suggests that balancing resting Ca²⁺ levels is a rather static process. However, direct ER Ca²⁺ imaging shows that resting Ca²⁺ levels are maintained by surprisingly dynamic Ca²⁺ fluxes between the ER Ca²⁺ store, the cytosol, and the extracellular space. The data show that the ER Ca²⁺ leak, continuously fed by the high-energy consuming SERCA, is a fundamental driver of resting Ca²⁺ dynamics. Based on simplistic Ca²⁺ toolkit models, we discuss how the ER Ca²⁺ leak could contribute to evolutionarily conserved Ca²⁺ phenomena such as Ca²⁺ entry, ER Ca²⁺ release, and Ca²⁺ oscillations.

Assessing K+ ions and K+ channel functions in cancer cell metabolism using fluorescent biosensors

Cancer represents a leading cause of death worldwide. Hence, a better understanding of the molecular mechanisms causing and propelling the disease is of utmost importance. Several cancer entities are associated with altered K⁺ channel expression which is frequently decisive for malignancy and disease outcome. The impact of such oncogenic K⁺ channels on cell patho-/physiology and homeostasis and their roles in different subcellular compartments is, however, far from being understood. A refined method to simultaneously investigate metabolic and ionic signaling events on the level of individual cells and their organelles represent genetically encoded fluorescent biosensors, that allow a high-resolution investigation of compartmentalized metabolite or ion dynamics in a non-invasive manner. This feature of these probes makes them versatile tools to visualize and understand subcellular consequences of aberrant K⁺ channel expression and activity in K⁺ channel related cancer research.

Homeostatic calcium fluxes, ER calcium release, SOCE, and calcium oscillations in cultured astrocytes are interlinked by a small calcium toolkit

How homeostatic ER calcium fluxes shape cellular calcium signals is still poorly understood. Here we used dual-color calcium imaging (ER-cytosol) and transcriptome analysis to link candidates of the calcium toolkit of astrocytes with homeostatic calcium signals. We found molecular and pharmacological evidence that P/Q-type channel Cacna1a contributes to depolarization-dependent calcium entry in astrocytes. For stimulated ER calcium release, the cells express the phospholipase Cb3, IP₃ receptors Itpr1 and Itpr2, but no ryanodine receptors (Ryr1-3). After IP₃-induced calcium release, Stim1/2 - Orai1/2/3 most likely mediate SOCE. The Serca2 (Atp2a2) is the candidate for refilling of the ER calcium store. The cells highly express adenosine receptor Adora1a for IP₃-induced calcium release. Accordingly, adenosine induces fast ER calcium release and subsequent ER calcium oscillations. After stimulation, calcium refilling of the ER depends on extracellular calcium. In response to SOCE, astrocytes show calcium-induced calcium release, notably even after ER calcium was depleted by extracellular calcium removal in unstimulated cells. In contrast, spontaneous ER-cytosol calcium oscillations were not fully dependent on extracellular calcium, as ER calcium oscillations could persist over minutes in calcium-free solution. Additionally, cell-autonomous calcium oscillations show a second-long spatial and temporal delay in the signal dynamics of ER and cytosolic calcium. Our data reveal a rather strong contribution of homeostatic calcium fluxes in shaping IP₃-induced and calcium-induced calcium release as well as spatiotemporal components of intracellular calcium oscillations.

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.

Tunable Organelle Imaging by Rational Design of Carbon Dots and Utilization of Uptake Pathways

Employing one-step hydrothermal treatment of o-phenylenediamine and lysine to exploit their self- and copolymerization, four kinds of CDs (ECDs, NCDs, GCDs, and LCDs) are synthesized, possessing different surface groups (CH₃, C-O-C, NH₂, and COOH) and lipophilicity which endow them with various uptake pathways to achieve tunable organelle imaging. Specifically, highly lipophilic ECDs with CH₃ group and NCDs with C-O-C group select passive manner to target to endoplasmic reticulum and nucleus, respectively. Amphiphilic GCDs with CH₃, C-O-C and NH₂ groups prefer caveolin-mediated endocytosis to locate at Golgi apparatus. Highly hydrophilic LCDs with CH₃, NH₂ and COOH groups are involved in clathrin-mediated endocytosis to localize in lysosomes. Besides, imaging results of cell division, three-dimensional reconstruction and living zebrafish demonstrate that the obtained CDs are promising potential candidates for specific organelle-targeting imaging.

The yin and yang of mitochondrial Ca2+ signaling in cell physiology and pathology

Mitochondria are autonomous and dynamic cellular organelles orchestrating a diverse range of cellular activities. Numerous cell-signaling pathways target these organelles and Ca²⁺ is one of the most significant. Mitochondria are able to rapidly and transiently take up Ca²⁺, thanks to the mitochondrial Ca²⁺ uniporter complex, as well as to extrude it through the Na⁺/Ca²⁺ and H⁺/Ca²⁺ exchangers. The transient accumulation of Ca²⁺ in the mitochondrial matrix impacts on mitochondrial functions and cell pathophysiology. Here we summarize the role of mitochondrial Ca²⁺ signaling in both physiological (yang) and pathological (yin) processes and the methods that can be used to investigate mitochondrial Ca²⁺ homeostasis. As an example of the pivotal role of mitochondria in pathology, we described the state of the art of mitochondrial Ca²⁺ alterations in different pathological conditions, with a special focus on Alzheimer's disease.

Luminescence Activity Decreases When v-coelenterazine Replaces Coelenterazine in Calcium-Regulated Photoprotein-A Theoretical and Experimental Study

Calcium-regulated photoproteins are found in at least five phyla of organisms. The light emitted by those photoproteins can be tuned by mutating the photoprotein and/or by modifying the substrate coelenterazine (CTZ). Thirty years ago, Shimomura observed that the luminescence activity of aequorin was dramatically reduced when the substrate CTZ was replaced by its analog v-CTZ. The latter is formed by adding a phenyl ring to the π-conjugated moiety of CTZ. The decrease in luminescence activity has not been understood until now. In this paper, through combined quantum mechanics and molecular mechanics calculations as well as molecular dynamics simulations, we discovered the reason for this observation. Modification of the substrate changes the conformation of nearby aromatic residues and enhances the π-π stacking interactions between the conjugated moiety of v-CTZ and the residues, which weakens the charge transfer to form light emitter and leads to a lower luminescence activity. The microenvironments of CTZ in obelin and in aequorin are very similar, so we predicted that the luminescence activity of obelin will also dramatically decrease when CTZ is replaced by v-CTZ. This prediction has received strong evidence from currently theoretical calculations and has been verified by experiments.

Direct monitoring of ER Ca2+ dynamics reveals that Ca2+ entry induces ER-Ca2+ release in astrocytes

Excitability in astroglia is controlled by Ca²⁺ fluxes from intracellular organelles, mostly from the endoplasmic reticulum (ER). Astrocytic ER possesses inositol 1,4,5-trisphosphate receptors (InsP₃R) that can be activated upon stimulation through a vast number of metabotropic G-protein-coupled receptors. By contrast, the role of Ca²⁺-gated Ca²⁺ release channels is less explored in astroglia. Here we address this process by monitoring Ca²⁺ dynamics directly in the cytosol and the ER of astroglial cells. Cultured astrocytes exhibited spontaneous and high-K-evoked cytosolic Ca²⁺ transients, both of them reversibly abolished by external Ca²⁺ removal, addition of plasma membrane channel blockers or ER Ca²⁺ depletion with SERCA inhibitors. Resting astrocyte [Ca²⁺]_ER averaged 400 μM and maximal stimulation with ATP provoked a complete and reversible ER discharge. Direct monitoring of Ca²⁺ in the lumen of ER showed that high-K induced a Ca²⁺ release from the ER, and its amplitude was proportional to the [K]. Furthermore, by combining the low affinity GAP3 indicator targeted to the ER with the high affinity cytosolic Rhod-2, we simultaneously imaged ER- and cytosolic-Ca²⁺ signals, in astrocytes in culture and in situ. Plasma membrane Ca²⁺ entry triggered a fast ER Ca²⁺ release coordinated with an increase in cytosolic Ca²⁺. Thus, we identify a Ca²⁺-induced Ca²⁺-release (CICR) mechanism that is likely to participate in spontaneous astroglial oscillations, providing a graded amplification of the cytosolic Ca²⁺ signal.

Sarcoplasmic reticulum Ca2+ decreases with age and correlates with the decline in muscle function in Drosophila

Sarcopenia, the loss of muscle mass and strength associated with age, has been linked to impairment of the cytosolic Ca2+ peak that triggers muscle contraction, but mechanistic details remain unknown. Here we explore the hypothesis that a reduction in sarcoplasmic reticulum (SR) Ca2+ concentration ([Ca2+]SR) is at the origin of this loss of Ca2+ homeostasis. We engineered Drosophila melanogaster to express the Ca2+ indicator GAP3 targeted to muscle SR, and we developed a new method to calibrate the signal into [Ca2+]SRin vivo [Ca2+]SR fell with age from ∼600 µM to 50 µM in close correlation with muscle function, which declined monotonically when [Ca2+]SR was <400 µM. [Ca2+]SR results from the pump-leak steady state at the SR membrane. However, changes in expression of the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump and of the ryanodine receptor leak were too modest to explain the large changes seen in [Ca2+]SR Instead, these changes are compatible with increased leakiness through the ryanodine receptor as the main determinant of the [Ca2+]SR decline in aging muscle. In contrast, there were no changes in endoplasmic reticulum [Ca2+] with age in brain neurons.This article has an associated First Person interview with the first author of the paper.

Live Mitochondrial or Cytosolic Calcium Imaging Using Genetically-encoded Cameleon Indicator in Mammalian Cells

Calcium (Ca²⁺) imaging aims at investigating the dynamic changes in live cells of its concentration ([Ca²⁺]) in different pathophysiological conditions. Ca²⁺ is an ubiquitous and versatile intracellular signal that modulates a large variety of cellular functions thanks to a cell type-specific toolkit and a complex subcellular compartmentalization. Many Ca²⁺ sensors are presently available (chemical and genetically encoded) that can be specifically targeted to different cellular compartments. Using these probes, it is now possible to monitor Ca²⁺ dynamics of living cells not only in the cytosol but also within specific organelles. The choice of a specific sensor depends on the experimental design and the spatial and temporal resolution required. Here we describe the use of novel Förster resonance energy transfer (FRET)-based fluorescent Ca²⁺ probes to dynamically and quantitatively monitor the changes in cytosolic and mitochondrial [Ca²⁺] in a variety of cell types and experimental conditions. FRET-based sensors have the enormous advantage of being ratiometric, a feature that makes them particularly suitable for quantitative and in vivo applications.

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.

Differential Effect of Glucose on ER-Mitochondria Ca2+ Exchange Participates in Insulin Secretion and Glucotoxicity-Mediated Dysfunction of β-Cells

Glucotoxicity-induced β-cell dysfunction in type 2 diabetes is associated with alterations of mitochondria and the endoplasmic reticulum (ER). Both organelles interact at contact sites, defined as mitochondria-associated membranes (MAMs), which were recently implicated in the regulation of glucose homeostasis. The role of MAMs in β-cells is still largely unknown, and their implication in glucotoxicity-associated β-cell dysfunction remains to be defined. Here, we report that acute glucose treatment stimulated ER-mitochondria interactions and calcium (Ca²⁺) exchange in INS-1E cells, whereas disruption of MAMs altered glucose-stimulated insulin secretion (GSIS). Conversely, chronic incubations with high glucose of either INS-1E cells or human pancreatic islets altered GSIS and concomitantly reduced ER Ca²⁺ store, increased basal mitochondrial Ca²⁺, and reduced ATP-stimulated ER-mitochondria Ca²⁺ exchanges, despite an increase of organelle interactions. Furthermore, glucotoxicity-induced perturbations of Ca²⁺ signaling are associated with ER stress, altered mitochondrial respiration, and mitochondria fragmentation, and these organelle stresses may participate in increased organelle tethering as a protective mechanism. Last, sustained induction of ER-mitochondria interactions using a linker reduced organelle Ca²⁺ exchange, induced mitochondrial fission, and altered GSIS. Therefore, dynamic organelle coupling participates in GSIS in β-cells, and over time, disruption of organelle Ca²⁺ exchange might be a novel mechanism contributing to glucotoxicity-induced β-cell dysfunction.

Genetically Encoded Calcium Indicators: A New Tool in Renal Hypertension Research

Hypertension is ranked as the third cause of disability-adjusted life-years. The percentage of the population suffering from hypertension will continue to increase over the next years. Renovascular disease is one of the most common causes of secondary hypertension. Vascular changes seen in hypertension are partially based on dysfunctional calcium signaling. This signaling can be studied using calcium indicators (loading dyes and genetically encoded calcium indicators; GECIs). Most progress in development has been seen in GECIs, which are used in an increasing number of publications concerning calcium signaling in vasculature and the kidney. The use of transgenic mouse models expressing GECIs will facilitate new possibilities to study dysfunctional calcium signaling in a cell type-specific manner, thus helping to identify more specific targets for treatment of (renal) hypertension.

Antimicrobial activity of Eucalyptus camaldulensis Dehn. plant extracts and essential oils: A review

Eucalyptus has become one of the world's most widely planted genera and E. camaldulensis (The River Red Gum) is a plantation species in many parts of the world. The plant traditional medical application indicates great antimicrobial properties, so E. camaldulensis essential oils and plant extracts have been widely examined. Essential oil of E. camaldulensis is active against many Gram positive (0.07-1.1%) and Gram negative bacteria (0.01-3.2%). The antibacterial effect is confirmed for bark and leaf extracts (conc. from 0.08 μg/mL to 200 mg/mL), with significant variations depending on extraction procedure. Eucalyptus camaldulensis essential oil and extracts are among the most active against bacteria when compared with those from other species of genus Eucalyptus. The most fungal model organisms are sensitive to 0.125-1.0% of E. camaldulensis essential oil. The extracts are active against C. albicans (0.2-200 mg/mL leaf extracts and 0.5 mg/mL bark extracts), and against various dermatophytes. Of particular importance is considerable the extracts' antiviral activity against animal and human viruses (0.1-50 μg/mL). Although the antiprotozoal activity of E. camaldulensis essential oil and extracts is in the order of magnitude of concentration several hundred mg/mL, it is considerable when taking into account current therapy cost, toxicity, and protozoal growing resistance. Some studies show that essential oils' and extracts' antimicrobial activity can be further potentiated in combinations with antibiotics (beta-lactams, fluorochinolones, aminoglycosides, polymyxins), antivirals (acyclovir), and extracts of other plants (e.g. Annona senegalensis; Psidium guajava). The present data confirm the river red gum considerable antimicrobial properties, which should be further examined with particular attention to the mechanisms of antimicrobial activity.

Filling a GAP-An Optimized Probe for ER Ca(2+) Imaging In Vivo

In this issue of Cell Chemical Biology, Navas-Navarro et al. (2016) demonstrate that fusion of engineered derivatives of the long-known jellyfish proteins green fluorescent protein (GFP) and aequorin yield optimized genetically encoded fluorescent probes for detecting Ca(2+) signals within the endoplasmic reticulum (ER) of living animals.

Fluorescent probes for organelle-targeted bioactive species imaging

The dynamic fluctuations of bioactive species in living cells are associated with numerous physiological and pathological phenomena. The emergence of organelle-targeted fluorescent probes has significantly facilitated our understanding on the biological functions of these species. This review describes the design, applications, challenges and potential directions of organelle-targeted bioactive species probes.

Bioactive species, including reactive oxygen species (ROS, including O₂˙^–, H₂O₂, HOCl, ¹O₂, ˙OH, HOBr, etc.), reactive nitrogen species (RNS, including ONOO^–, NO, NO₂, HNO, etc.), reactive sulfur species (RSS, including GSH, Hcy, Cys, H₂S, H₂S_n, SO₂ derivatives, etc.), ATP, HCHO, CO and so on, are a highly important category of molecules in living cells. The dynamic fluctuations of these molecules in subcellular microenvironments determine cellular homeostasis, signal conduction, immunity and metabolism. However, their abnormal expressions can cause disorders which are associated with diverse major diseases. Monitoring bioactive molecules in subcellular structures is therefore critical for bioanalysis and related drug discovery. With the emergence of organelle-targeted fluorescent probes, significant progress has been made in subcellular imaging. Among the developed subcellular localization fluorescent tools, ROS, RNS and RSS (RONSS) probes are highly attractive, owing to their potential for revealing the physiological and pathological functions of these highly reactive, interactive and interconvertible molecules during diverse biological events, which are rather significant for advancing our understanding of different life phenomena and exploring new technologies for life regulation. This review mainly illustrates the design principles, detection mechanisms, current challenges, and potential future directions of organelle-targeted fluorescent probes toward RONSS.

Mitochondrial Ca2+ concentrations in live cells: quantification methods and discrepancies

Intracellular Ca²⁺ signaling controls numerous cellular functions. Mitochondria respond to cytosolic Ca²⁺ changes by adapting mitochondrial functions and, in some cell types, shaping the spatiotemporal properties of the cytosolic Ca²⁺ signal. Numerous methods have been developed to specifically and quantitatively measure the mitochondrial-free Ca²⁺ concentrations ([Ca²⁺ ]_m ), but there are still significant discrepancies in the calculated absolute values of [Ca²⁺ ]_m in stimulated live cells. These discrepancies may be due to the distinct properties of the methods used to measure [Ca²⁺ ]_m , the calcium-free/bound ratio, and the cell-type and stimulus-dependent Ca²⁺ dynamics. Critical processes happening in the mitochondria, such as ATP generation, ROS homeostasis, and mitochondrial permeability transition opening, depend directly on the [Ca²⁺ ]_m values. Thus, precise determination of absolute [Ca²⁺ ]_m values is imperative for understanding Ca²⁺ signaling. This review summarizes the reported calibrated [Ca²⁺ ]_m values in many cell types and discusses the discrepancies among these values. Areas for future research are also proposed.

Genetically Encoded Fluorescent Biosensors Illuminate the Spatiotemporal Regulation of Signaling Networks

Cellular signaling networks are the foundation which determines the fate and function of cells as they respond to various cues and stimuli. The discovery of fluorescent proteins over 25 years ago enabled the development of a diverse array of genetically encodable fluorescent biosensors that are capable of measuring the spatiotemporal dynamics of signal transduction pathways in live cells. In an effort to encapsulate the breadth over which fluorescent biosensors have expanded, we endeavored to assemble a comprehensive list of published engineered biosensors, and we discuss many of the molecular designs utilized in their development. Then, we review how the high temporal and spatial resolution afforded by fluorescent biosensors has aided our understanding of the spatiotemporal regulation of signaling networks at the cellular and subcellular level. Finally, we highlight some emerging areas of research in both biosensor design and applications that are on the forefront of biosensor development.

Caffeine chelates calcium in the lumen of the endoplasmic reticulum

Cytosolic Ca²⁺ signals are often amplified by massive calcium release from the endoplasmic reticulum (ER). This calcium-induced calcium release (CICR) occurs by activation of an ER Ca²⁺ channel, the ryanodine receptor (RyR), which is facilitated by both cytosolic- and ER Ca²⁺ levels. Caffeine sensitizes RyR to Ca²⁺ and promotes ER Ca²⁺ release at basal cytosolic Ca²⁺ levels. This outcome is frequently used as a readout for the presence of CICR. By monitoring ER luminal Ca²⁺ with the low-affinity genetic Ca²⁺ probe erGAP3, we find here that application of 50 mM caffeine rapidly reduces the Ca²⁺ content of the ER in HeLa cells by ∼50%. Interestingly, this apparent ER Ca²⁺ release does not go along with the expected cytosolic Ca²⁺ increase. These results can be explained by Ca²⁺ chelation by caffeine inside the ER. Ca²⁺-overloaded mitochondria also display a drop of the matrix Ca²⁺ concentration upon caffeine addition. In contrast, in the cytosol, with a low free Ca²⁺ concentration (10^-7 M), no chelation is observed. Expression of RyR3 sensitizes the responses to caffeine with effects both in the ER (increase in Ca²⁺ release) and in the cytosol (increase in Ca²⁺ peak) at low caffeine concentrations (0.3-1 mM) that have no effects in control cells. Our results illustrate the fact that simultaneous monitoring of both cytosolic- and ER Ca²⁺ are necessary to understand the action of caffeine and raise concerns against the use of high concentrations of caffeine as a readout of the presence of CICR.

Role of Endoplasmic Reticulum-Mediated Ca2+ Signaling in Neuronal Cell Death

SIGNIFICANCE

Properly controlled intracellular Ca²⁺ dynamics is crucial for regulation of neuronal function and survival in the central nervous system. The endoplasmic reticulum (ER), a major intracellular Ca²⁺ store, plays a critical role as a source and sink for neuronal Ca²⁺. Recent Advances: Accumulating evidence indicates that disrupted ER Ca²⁺ signaling is involved in neuronal cell death under various pathological conditions, providing novel insight into neurodegenerative disease mechanisms.

CRITICAL ISSUES

We summarize current knowledge concerning the relationship between abnormal ER Ca²⁺ dynamics and neuronal cell death. We also introduce recent technical advances for probing ER intraluminal Ca²⁺ dynamics with unprecedented spatiotemporal resolution.

FUTURE DIRECTIONS

Further studies on ER Ca²⁺ signaling are expected to provide progress for unmet medical needs in neurodegenerative disease. Antioxid. Redox Signal. 29, 1147-1157.

The machineries, regulation and cellular functions of mitochondrial calcium

Calcium ions (Ca²⁺) are some of the most versatile signalling molecules, and they have many physiological functions, prominently including muscle contraction, neuronal excitability, cell migration and cell growth. By sequestering and releasing Ca²⁺, mitochondria serve as important regulators of cellular Ca²⁺. Mitochondrial Ca²⁺ also has other important functions, such as regulation of mitochondrial metabolism, ATP production and cell death. In recent years, identification of the molecular machinery regulating mitochondrial Ca²⁺ accumulation and efflux has expanded the number of (patho)physiological conditions that rely on mitochondrial Ca²⁺ homeostasis. Thus, expanding the understanding of the mechanisms of mitochondrial Ca²⁺ regulation and function in different cell types is an important task in biomedical research, which offers the possibility of targeting mitochondrial Ca²⁺ machinery for the treatment of several disorders.

Dual Color Sensors for Simultaneous Analysis of Calcium Signal Dynamics in the Nuclear and Cytoplasmic Compartments of Plant Cells

Spatiotemporal changes in cellular calcium (Ca²⁺) concentrations are essential for signal transduction in a wide range of plant cellular processes. In legumes, nuclear and perinuclear-localized Ca²⁺ oscillations have emerged as key signatures preceding downstream symbiotic signaling responses. Förster resonance energy transfer (FRET) yellow-based Ca²⁺ cameleon probes have been successfully exploited to measure the spatiotemporal dynamics of symbiotic Ca²⁺ signaling in legumes. Although providing cellular resolution, these sensors were restricted to measuring Ca²⁺ changes in single subcellular compartments. In this study, we have explored the potential of single fluorescent protein-based Ca²⁺ sensors, the GECOs, for multicolor and simultaneous imaging of the spatiotemporal dynamics of cytoplasmic and nuclear Ca²⁺ signaling in root cells. Single and dual fluorescence nuclear and cytoplasmic-localized GECOs expressed in transgenic Medicago truncatula roots and Arabidopsis thaliana were used to successfully monitor Ca²⁺ responses to microbial biotic and abiotic elicitors. In M. truncatula, we demonstrate that GECOs detect symbiosis-related Ca²⁺ spiking variations with higher sensitivity than the yellow FRET-based sensors previously used. Additionally, in both M. truncatula and A. thaliana, the dual sensor is now able to resolve in a single root cell the coordinated spatiotemporal dynamics of nuclear and cytoplasmic Ca²⁺ signaling in vivo. The GECO-based sensors presented here therefore represent powerful tools to monitor Ca²⁺ signaling dynamics in vivo in response to different stimuli in multi-subcellular compartments of plant cells.

Optogenetic Tools for Subcellular Applications in Neuroscience

The ability to study cellular physiology using photosensitive, genetically encoded molecules has profoundly transformed neuroscience. The modern optogenetic toolbox includes fluorescent sensors to visualize signaling events in living cells and optogenetic actuators enabling manipulation of numerous cellular activities. Most optogenetic tools are not targeted to specific subcellular compartments but are localized with limited discrimination throughout the cell. Therefore, optogenetic activation often does not reflect context-dependent effects of highly localized intracellular signaling events. Subcellular targeting is required to achieve more specific optogenetic readouts and photomanipulation. Here we first provide a detailed overview of the available optogenetic tools with a focus on optogenetic actuators. Second, we review established strategies for targeting these tools to specific subcellular compartments. Finally, we discuss useful tools and targeting strategies that are currently missing from the optogenetics repertoire and provide suggestions for novel subcellular optogenetic applications.

Type 3 inositol 1,4,5-trisphosphate receptor is dispensable for sensory activation of the mammalian vomeronasal organ

Signal transduction in sensory neurons of the mammalian vomeronasal organ (VNO) involves the opening of the canonical transient receptor potential channel Trpc2, a Ca²⁺-permeable cation channel that is activated by diacylglycerol and inhibited by Ca²⁺-calmodulin. There has been a long-standing debate about the extent to which the second messenger inositol 1,4,5-trisphosphate (InsP₃) and type 3 InsP₃ receptor (InsP₃R3) are involved in the opening of Trpc2 channels and in sensory activation of the VNO. To address this question, we investigated VNO function of mice carrying a knockout mutation in the Itpr3 locus causing a loss of InsP₃R3. We established a new method to monitor Ca²⁺ in the endoplasmic reticulum of vomeronasal sensory neurons (VSNs) by employing the GFP-aequorin protein sensor erGAP2. We also performed simultaneous InsP₃ photorelease and Ca²⁺ monitoring experiments, and analysed Ca²⁺ dynamics, sensory currents, and action potential or field potential responses in InsP₃R3-deficient VSNs. Disruption of Itpr3 abolished or minimized the Ca²⁺ transients evoked by photoactivated InsP₃, but there was virtually no effect on sensory activation of VSNs. Therefore, InsP₃R3 is dispensable for primary chemoelectrical transduction in mouse VNO. We conclude that InsP₃R3 is not required for gating of Trpc2 in VSNs.

Bioluminescence Assays for Monitoring Chondrogenic Differentiation and Cartilage Regeneration

Since articular cartilage has a limited regeneration potential, for developing biological therapies for cartilage regeneration it is important to study the mechanisms underlying chondrogenesis of stem cells. Bioluminescence assays can visualize a wide range of biological phenomena such as gene expression, signaling, metabolism, development, cellular movements, and molecular interactions by using visible light and thus contribute substantially to elucidation of their biological functions. This article gives a concise review to introduce basic principles of bioluminescence assays and applications of the technology to visualize the processes of chondrogenesis and cartilage regeneration. Applications of bioluminescence assays have been highlighted in the methods of real-time monitoring of gene expression and intracellular levels of biomolecules and noninvasive cell tracking within animal models. This review suggests that bioluminescence assays can be applied towards a visual understanding of chondrogenesis and cartilage regeneration.

Variability of fluorescence spectra of coelenteramide-containing proteins as a basis for toxicity monitoring

Nowadays, physicochemical approach to understanding toxic effects remains underdeveloped. A proper development of such mode would be concerned with simplest bioassay systems. Coelenteramide-Containing Fluorescent Proteins (CLM-CFPs) can serve as proper tools for study primary physicochemical processes in organisms under external exposures. CLM-CFPs are products of bioluminescent reactions of marine coelenterates. As opposed to Green Fluorescent Proteins, the CLM-CFPs are not widely applied in biomedical research, and their potential as colored biomarkers is undervalued now. Coelenteramide, fluorophore of CLM-CFPs, is a photochemically active molecule; it acts as a proton donor in its electron-excited states, generating several forms of different fluorescent state energy and, hence, different fluorescence color, from violet to green. Contributions of the forms to the visible fluorescence depend on the coelenteramide microenvironment in proteins. Hence, CLM-CFPs can serve as fluorescence biomarkers with color differentiation to monitor results of destructive biomolecule exposures. The paper reviews experimental and theoretical studies of spectral-luminescent and photochemical properties of CLM-CFPs, as well as their variation under different exposures - chemicals, temperature, and ionizing radiation. Application of CLM-CFPs as toxicity bioassays of a new type is justified.

Genetically Encoded Calcium Indicators as Probes to Assess the Role of Calcium Channels in Disease and for High-Throughput Drug Discovery

The calcium ion (Ca²⁺) is an important signaling molecule implicated in many cellular processes, and the remodeling of Ca²⁺ homeostasis is a feature of a variety of pathologies. Typical methods to assess Ca²⁺ signaling in cells often employ small molecule fluorescent dyes, which are sometimes poorly suited to certain applications such as assessment of cellular processes, which occur over long periods (hours or days) or in vivo experiments. Genetically encoded calcium indicators are a set of tools available for the measurement of Ca²⁺ changes in the cytosol and subcellular compartments, which circumvent some of the inherent limitations of small molecule Ca²⁺ probes. Recent advances in genetically encoded calcium sensors have greatly increased their ability to provide reliable monitoring of Ca²⁺ changes in mammalian cells. New genetically encoded calcium indicators have diverse options in terms of targeting, Ca²⁺ affinity and fluorescence spectra, and this will further enhance their potential use in high-throughput drug discovery and other assays. This review will outline the methods available for Ca²⁺ measurement in cells, with a focus on genetically encoded calcium sensors. How these sensors will improve our understanding of the deregulation of Ca²⁺ handling in disease and their application to high-throughput identification of drug leads will also be discussed.

Fluorescent Protein-photoprotein Fusions and Their Applications in Calcium Imaging

Calcium-activated photoproteins, such as aequorin, have been used as luminescent Ca²⁺ indicators since 1967. After the cloning of aequorin in 1985, microinjection was substituted by its heterologous expression, which opened the way for a widespread use. Molecular fusion of green fluorescent protein (GFP) to aequorin recapitulated the nonradiative energy transfer process that occurs in the jellyfish Aequorea victoria, from which these two proteins were obtained, resulting in an increase of light emission and a shift to longer wavelength. The abundance and location of the chimera are seen by fluorescence, whereas its luminescence reports Ca²⁺ levels. GFP-aequorin is broadly used in an increasing number of studies, from organelles and cells to intact organisms. By fusing other fluorescent proteins to aequorin, the available luminescence color palette has been expanded for multiplexing assays and for in vivo measurements. In this report, we will attempt to review the various photoproteins available, their reported fusions with fluorescent proteins and their biological applications to image Ca²⁺ dynamics in organelles, cells, tissue explants and in live organisms.

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.

Measuring Ca2+ inside intracellular organelles with luminescent and fluorescent aequorin-based sensors

GFP-Aequorin Protein (GAP) can be used to measure [Ca²⁺] inside intracellular organelles, both by luminescence and by fluorescence. The low-affinity variant GAP3 is adequate for ratiometric imaging in the endoplasmic reticulum and Golgi apparatus, and it can be combined with conventional synthetic indicators for simultaneous measurements of cytosolic Ca²⁺. GAP is bioorthogonal as it does not have mammalian homologues, and it is robust and functionally expressed in transgenic flies and mice, where it can be used for Ca²⁺ measurements ex vivo and in vivo to explore animal models of health and disease. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Organelle-Specific Sensors for Monitoring Ca2+ Dynamics in Neurons

Calcium (Ca²⁺) plays innumerable critical functions in neurons ranging from regulation of neurotransmitter release and synaptic plasticity to activity-dependent transcription. Therefore, more than any other cell types, neurons are critically dependent on spatially and temporally controlled Ca²⁺ dynamics. This is achieved through an exquisite level of compartmentalization of Ca²⁺ storage and release from various organelles. The function of these organelles in the regulation of Ca²⁺ dynamics has been studied for decades using electrophysiological and optical methods combined with pharmacological and genetic alterations. Mitochondria and the endoplasmic reticulum (ER) are among the organelles playing the most critical roles in Ca²⁺ dynamics in neurons. At presynaptic boutons, Ca²⁺ triggers neurotransmitter release and synaptic plasticity, and postsynaptically, Ca²⁺ mobilization mediates long-term synaptic plasticity. To explore Ca²⁺ dynamics in live cells and intact animals, various synthetic and genetically encoded fluorescent Ca²⁺ sensors were developed, and recently, many groups actively increased the sensitivity and diversity of genetically encoded Ca²⁺ indicators (GECIs). Following conjugation with various signal peptides, these improved GECIs can be targeted to specific subcellular compartments, allowing monitoring of organelle-specific Ca²⁺ dynamics. Here, we review recent findings unraveling novel roles for mitochondria- and ER-dependent Ca²⁺ dynamics in neurons and at synapses.

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.

GFP-Aequorin Protein Sensor for Ex Vivo and In Vivo Imaging of Ca(2+) Dynamics in High-Ca(2+) Organelles

Proper functioning of organelles such as the ER or the Golgi apparatus requires luminal accumulation of Ca(2+) at high concentrations. Here we describe a ratiometric low-affinity Ca(2+) sensor of the GFP-aequorin protein (GAP) family optimized for measurements in high-Ca(2+) concentration environments. Transgenic animals expressing the ER-targeted sensor allowed monitoring of Ca(2+) signals inside the organelle. The use of the sensor was demonstrated under three experimental paradigms: (1) ER Ca(2+) oscillations in cultured astrocytes, (2) ex vivo functional mapping of cholinergic receptors triggering ER Ca(2+) release in acute hippocampal slices from transgenic mice, and (3) in vivo sarcoplasmic reticulum Ca(2+) dynamics in the muscle of transgenic flies. Our results provide proof of the suitability of the new biosensors to monitor Ca(2+) dynamics inside intracellular organelles under physiological conditions and open an avenue to explore complex Ca(2+) signaling in animal models of health and disease.

Visualization of Ca2+ Filling Mechanisms upon Synaptic Inputs in the Endoplasmic Reticulum of Cerebellar Purkinje Cells

The endoplasmic reticulum (ER) plays crucial roles in intracellular Ca(2+) signaling, serving as both a source and sink of Ca(2+), and regulating a variety of physiological and pathophysiological events in neurons in the brain. However, spatiotemporal Ca(2+) dynamics within the ER in central neurons remain to be characterized. In this study, we visualized synaptic activity-dependent ER Ca(2+) dynamics in mouse cerebellar Purkinje cells (PCs) using an ER-targeted genetically encoded Ca(2+) indicator, G-CEPIA1er. We used brief parallel fiber stimulation to induce a local decrease in the ER luminal Ca(2+) concentration ([Ca(2+)]ER) in dendrites and spines. In this experimental system, the recovery of [Ca(2+)]ER takes several seconds, and recovery half-time depends on the extent of ER Ca(2+) depletion. By combining imaging analysis and numerical simulation, we show that the intraluminal diffusion of Ca(2+), rather than Ca(2+) reuptake, is the dominant mechanism for the replenishment of the local [Ca(2+)]ER depletion immediately following the stimulation. In spines, the ER filled almost simultaneously with parent dendrites, suggesting that the ER within the spine neck does not represent a significant barrier to Ca(2+) diffusion. Furthermore, we found that repetitive climbing fiber stimulation, which induces cytosolic Ca(2+) spikes in PCs, cumulatively increased [Ca(2+)]ER. These results indicate that the neuronal ER functions both as an intracellular tunnel to redistribute stored Ca(2+) within the neurons, and as a leaky integrator of Ca(2+) spike-inducing synaptic inputs.

Enabling Aequorin for Biotechnology Applications Through Genetic Engineering

In recent years, luminescent proteins have been studied for their potential application in a variety of detection systems. Bioluminescent proteins, which do not require an external excitation source, are especially well-suited as reporters in analytical detection. The photoprotein aequorin is a bioluminescent protein that can be engineered for use as a molecular reporter under a wide range of conditions while maintaining its sensitivity. Herein, the characteristics of aequorin as well as the engineering and production of aequorin variants and their impact on signal detection in biological systems are presented. The structural features and activity of aequorin, its benefits as a label for sensing and applications in highly sensitive detection, as well as in gaining insight into biological processes are discussed. Among those, focus has been placed on the highly sensitive calcium detection in vivo, in vitro DNA and small molecule sensing, and development of in vivo imaging technologies. Graphical Abstract.

A Low Affinity GCaMP3 Variant (GCaMPer) for Imaging the Endoplasmic Reticulum Calcium Store

Endoplasmic reticulum calcium homeostasis is critical for cellular functions and is disrupted in diverse pathologies including neurodegeneration and cardiovascular disease. Owing to the high concentration of calcium within the ER, studying this subcellular compartment requires tools that are optimized for these conditions. To develop a single-fluorophore genetically encoded calcium indicator for this organelle, we targeted a low affinity variant of GCaMP3 to the ER lumen (GCaMPer (10.19)). A set of viral vectors was constructed to express GCaMPer in human neuroblastoma cells, rat primary cortical neurons, and human induced pluripotent stem cell-derived cardiomyocytes. We observed dynamic changes in GCaMPer (10.19) fluorescence in response to pharmacologic manipulations of the ER calcium store. Additionally, periodic calcium efflux from the ER was observed during spontaneous beating of cardiomyocytes. GCaMPer (10.19) has utility in imaging ER calcium in living cells and providing insight into luminal calcium dynamics under physiologic and pathologic states.

A new low-Ca²⁺ affinity GAP indicator to monitor high Ca²⁺ in organelles by luminescence

We have recently described a new class of genetically encoded Ca(2+) indicators composed of two jellyfish proteins, a variant of green fluorescent protein (GFP) and the calcium binding protein apoaequorin, named GAP (Rodriguez-García et al., 2014). GAP is a unique dual-mode Ca(2+) indicator, able to function either as a fluorescent or a luminescent probe, depending on whether the photoprotein aequorin is in its apo-state or reconstituted with its cofactor coelenterazine. We describe here a novel application of GAP as a low affinity bioluminescent indicator, suitable for measurements of [Ca(2+)] in ER or in Golgi apparatus. We used the low affinity variant, GAP1, which carries mutations in two EF-hands of aequorin, reconstituted with coelenterazine n. In comparison to previous bioluminescent aequorin fusions, the decay rate of GAP1 was decreased 8 fold and the affinity for Ca(2+) was lowered one order of magnitude. This improvement allows long-term measurements in high Ca(2+) environments avoiding fast aequorin consumption. GAP1 was targeted to the ER of various cell types, where it monitored resting Ca(2+) concentrations in the range from 400 to 600 μM. ER could be emptied of calcium by stimulation with ATP, carbachol or histamine in intact cells, and by challenge with inositol tris-phosphate in permeabilized cells. GAP1 was also targeted to the Golgi apparatus where it was able to precisely monitor long-term calcium dynamics. GAP1 provides a novel and robust indicator applicable to bioluminescent high-throughput quantitative assays.

Ratiometric biological nanosensors

The measurement of intracellular analytes has been key in understanding cellular processes and function, and the use of biological nanosensors has revealed the spatial and temporal variation in their concentrations. In particular, ratiometric nanosensors allow quantitative measurements of analyte concentrations. The present review focuses on the recent advances in ratiometric intracellular biological nanosensors, with an emphasis on their utility in measuring analytes that are important in cell function.

Spying on organelle Ca²⁺ in living cells: the mitochondrial point of view

Over the past years, the use of genetically encoded Ca(2+) indicators (GECIs), derived from aequorin and green fluorescent protein, has profoundly transformed the study of Ca(2+) homeostasis in living cells leading to novel insights into functional aspects of Ca(2+) signalling. Particularly relevant for a deeper understanding of these key aspects of cell pathophysiology has been the possibility of imaging changes in Ca(2+) concentration not only in the cytoplasm, but also inside organelles. In this review, we will provide an overview of the ongoing developments in the use of GECIs, with particular focus on mitochondrially targeted probes. Indeed, due to recent advances in organelle Ca(2+) imaging with GECIs, mitochondria are now at the centre of renewed interest: they play key roles both in the physiology of the cell and in multiple pathological conditions relevant to human health.