Calcium dependence of aequorin bioluminescence dissected by random mutagenesis

Fluorescent Protein-Based Sensors for Detecting Essential Metal Ions across the Tree of Life

Genetically encoded fluorescent metal ion sensors are powerful tools for elucidating metal dynamics in living systems. Over the last 25 years since the first examples of genetically encoded fluorescent protein-based calcium indicators, this toolbox of probes has expanded to include other essential and non-essential metal ions. Collectively, these tools have illuminated fundamental aspects of metal homeostasis and trafficking that are crucial to fields ranging from neurobiology to human nutrition. Despite these advances, much of the application of metal ion sensors remains limited to mammalian cells and tissues and a limited number of essential metals. Applications beyond mammalian systems and in vivo applications in living organisms have primarily used genetically encoded calcium ion sensors. The aim of this Perspective is to provide, with the support of historical and recent literature, an updated and critical view of the design and use of fluorescent protein-based sensors for detecting essential metal ions in various organisms. We highlight the historical progress and achievements with calcium sensors and discuss more recent advances and opportunities for the detection of other essential metal ions. We also discuss outstanding challenges in the field and directions for future studies, including detecting a wider variety of metal ions, developing and implementing a broader spectral range of sensors for multiplexing experiments, and applying sensors to a wider range of single- and multi-species biological systems.

Longer characteristic wavelength in a novel engineered photoprotein Mnemiopsin 2

We designed two mutants of photoprotein Mnemiopsin 2 (Mn2) including M52I and V144I, where the mutations were applied in the EF-hand loops I and III. Far-UV CD measurements demonstrated that the stability of the helices in the wild-type (WT) protein is greater compared with the mutants. Heat-induced denaturation experiments in the apo-form of photoproteins showed that WT Mn2 has higher value of the enthalpy change for the unfolding process, indicating that it has more stabilizing interaction compared with mutants. According to the activity measurement data, both mutants, particularly V144I have lower initial intensity as well as slower decay rate as compared with the WT photoprotein. Importantly, it was found that V144I variant shows 25 nm of red shift in the characteristic wavelengths as compared with the WT photoprotein. This finding can be considered as an advantage for in vivo application of photoprotein for imaging purposes. It concluded that this position on loop III of Mn2 is a hotspot point for characteristic wavelength determination. However, further research on this mutant is needed for making stable variants of Mn2 with novel optical features.

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.

Sajedi R. Thermostability of Ctenophore and Coelenterate Ca2+-Regulated Apo-photoproteins: A Comparative Study

The stabilities of Ca²⁺-regulated ctenophore and coelenterate apo-photoproteins, apo-mnemiopsin (apo-Mne) and apo-aequorin (apo-Aeq), respectively, were compared biochemically, biophysically, and structurally. Despite high degrees of structural and functional conservation, drastic variations in stability and structural dynamics were found between the two proteins. Irreversible thermoinactivation experiments were performed upon incubation of apo-photoproteins at representative temperatures. The inactivation rate constants (k_inact) at 50 °C were determined to be 0.001 and 0.004 min^-1 for apo-Mne and apo-Aeq, respectively. Detailed analysis of the inactivation process suggests that the higher thermostability of apo-Mne is due to the higher activation energy (E_a) and subsequently higher values of ΔH* and ΔG* at a given temperature. According to molecular dynamics simulation studies, the higher hydrogen bond, electrostatic, and van der Waals energies in apo-Mne can validate the relationship between the thermal adaptation of apo-Mne and the energy barrier for the inactivation process. Our results show that favorable residues for protein thermostability such as hydrophobic, charged, and adopted α-helical structure residues are more frequent in the apo-Mne structure. Although the effect of acrylamide on fluorescence quenching suggests that the local flexibility in regions around Trp and Tyr residues of apo-Aeq is higher than that of apo-Mne, which results in it having a better ability to penetrate acrylamide molecules, the root-mean-square fluctuation of helix A in apo-Mne is higher than that in apo-Aeq. It seems that the greater flexibility of apo-Mne in these regions may be considered as a determining factor, affecting the thermal stability of apo-Mne through a balance between structural rigidity and flexibility.

RedquorinXS Mutants with Enhanced Calcium Sensitivity and Bioluminescence Output Efficiently Report Cellular and Neuronal Network Activities

Considerable efforts have been focused on shifting the wavelength of aequorin Ca²⁺-dependent blue bioluminescence through fusion with fluorescent proteins. This approach has notably yielded the widely used GFP-aequorin (GA) Ca²⁺ sensor emitting green light, and tdTomato-aequorin (Redquorin), whose bioluminescence is completely shifted to red, but whose Ca²⁺ sensitivity is low. In the present study, the screening of aequorin mutants generated at twenty-four amino acid positions in and around EF-hand Ca²⁺-binding domains resulted in the isolation of six aequorin single or double mutants (AequorinXS) in EF2, EF3, and C-terminal tail, which exhibited markedly higher Ca²⁺ sensitivity than wild-type aequorin in vitro. The corresponding Redquorin mutants all showed higher Ca²⁺ sensitivity than wild-type Redquorin, and four of them (RedquorinXS) matched the Ca²⁺ sensitivity of GA in vitro. RedquorinXS mutants exhibited unaltered thermostability and peak emission wavelengths. Upon stable expression in mammalian cell line, all RedquorinXS mutants reported the activation of the P2Y2 receptor by ATP with higher sensitivity and assay robustness than wt-Redquorin, and one, RedquorinXS-Q159T, outperformed GA. Finally, wide-field bioluminescence imaging in mouse neocortical slices showed that RedquorinXS-Q159T and GA similarly reported neuronal network activities elicited by the removal of extracellular Mg²⁺. Our results indicate that RedquorinXS-Q159T is a red light-emitting Ca²⁺ sensor suitable for the monitoring of intracellular signaling in a variety of applications in cells and tissues, and is a promising candidate for the transcranial monitoring of brain activities in living mice.

Bioluminescence calcium imaging of network dynamics and their cholinergic modulation in slices of cerebral cortex from male rats

The activity of neuronal ensembles was monitored in neocortical slices from male rats using wide-field bioluminescence imaging of a calcium sensor formed with the fusion of green fluorescent protein and aequorin (GA) and expressed through viral transfer. GA expression was restricted to pyramidal neurons and did not conspicuously alter neuronal morphology or neocortical cytoarchitecture. Removal of extracellular magnesium or addition of GABA receptor antagonists triggered epileptiform flashes of variable amplitude and spatial extent, indicating that the excitatory and inhibitory networks were functionally preserved in GA-expressing slices. We found that agonists of muscarinic acetylcholine receptors largely increased the peak bioluminescence response to local electrical stimulation in layer I or white matter, and gave rise to a slowly decaying response persisting for tens of seconds. The peak increase involved layers II/III and V and did not result in marked alteration of response spatial properties. The persistent response involved essentially layer V and followed the time course of the muscarinic afterdischarge depolarizing plateau in layer V pyramidal cells. This plateau potential triggered spike firing in layer V, but not layer II/III pyramidal cells, and was accompanied by recurrent synaptic excitation in layer V. Our results indicate that wide-field imaging of GA bioluminescence is well suited to monitor local and global network activity patterns, involving different mechanisms of intracellular calcium increase, and occurring on various timescales.

Glycosylation Pattern and in vitro Bioactivity of Reference Follitropin alfa and Biosimilars

Recombinant follicle-stimulating hormone (FSH) (follitropin alfa) and biosimilar preparations are available for clinical use. They have specific FSH activity and a unique glycosylation profile dependent on source cells. The aim of the study is to compare the originator (reference) follitropin alfa (Gonal-f®)- with biosimilar preparations (Bemfola® and Ovaleap®)-induced cellular responses in vitro. Gonadotropin N-glycosylation profiles were analyzed by ELISA lectin assay, revealing preparation specific-patterns of glycan species (Kruskal-Wallis test; p < 0.05, n = 6) and by glycotope mapping. Increasing concentrations of Gonal-f® or biosimilar (1 × 10^-3-1 × 10³ ng/ml) were used for treating human primary granulosa lutein cells (hGLC) and FSH receptor (FSHR)-transfected HEK293 cells in vitro. Intracellular cAMP production, Ca²⁺ increase and β-arrestin 2 recruitment were evaluated by BRET, CREB, and ERK1/2 phosphorylation by Western blotting. 12-h gene expression, and 8- and 24-h progesterone and estradiol synthesis were measured by real-time PCR and immunoassay, respectively. We found preparation-specific glycosylation patterns by lectin assay (Kruskal-Wallis test; p < 0.001; n = 6), and similar cAMP production and β-arrestin 2 recruitment in FSHR-transfected HEK293 cells (cAMP EC₅₀ range = 12 ± 0.9-24 ± 1.7 ng/ml; β-arrestin 2 EC₅₀ range = 140 ± 14.1-313 ± 18.7 ng/ml; Kruskal-Wallis test; p ≥ 0.05; n = 4). Kinetics analysis revealed that intracellular Ca²⁺ increased upon cell treatment by 4 μg/ml Gonal-f®, while equal concentrations of biosimilars failed to induced a response (Kruskal-Wallis test; p < 0.05; n = 3). All preparations induced both 8 and 24 h-progesterone and estradiol synthesis in hGLC, while no different EC₅₀s were demonstrated (Kruskal-Wallis test; p > 0.05; n = 5). Apart from preparation-specific intracellular Ca²⁺ increases achieved at supra-physiological hormone doses, all compounds induced similar intracellular responses and steroidogenesis, reflecting similar bioactivity, and overall structural homogeneity.

Multiplexing cytokine analysis: towards reducing sample volume needs in clinical diagnostics

Our work demonstrates the use of both spatial and temporal resolution to quantify multiple analytes based on bioluminescent labels.

Molecular mechanisms governing the evolutionary conservation of Glycine in the 6th position of loops ΙΙΙ and ΙV in photoprotein mnemiopsin 2

Photoproteins in their functional form are complexed noncovalently with 2-hydroperoxycoelenterazine. A conformational change upon coordination of Ca⁺² ions with their EF-hand loops leads to oxidation of substrate and emission of light. In all photoproteins, EF-hand loops Ι, ΙΙΙ and ΙV have standard sequence for binding to Ca⁺² ion, however the second one is not able for Ca⁺² coordination. Sequence analysis of Mnemiopsin 2 and other known photoproteins shows that Glutamate (Glu) is occurred in the 6th position of its first EF-hand loop, but this position in other loops of mnemiopsin 2 and all functional loops of other photoproteins is occupied by Glycine (Gly). Here we designed and made single and double mutants where Gly residue at the 6th positions of loops ΙΙΙ and ΙV of mnemiopsin 2 was replaced with Glu. According to the activity measurements, wild-type (WT) and G142E variants have more initial luminescence intensity than G176E and double mutants; while WT and G176E have higher values of half decay time when compared with G142E and double mutants. According to the isothermal denaturation experiments, all protein variants are structurally more stable than WT mnemiopsin 2 and that the stabilizing effects of single mutants are paired resulting in more stability of double mutant against urea denaturation. We concluded that simultaneous occurrence of Gly in the 6th position of loops ΙΙΙ and ΙV is essential for evolutionary adjustment of initial intensity and decay rate of luminescence emission via affecting the interaction of the core structure of photoprotein with coelenteramide and binding affinity of Ca⁺² to the corresponding loops, respectively.

Fluorescent Proteins for Investigating Biological Events in Acidic Environments

The interior lumen of acidic organelles (e.g., endosomes, secretory granules, lysosomes and plant vacuoles) is an important platform for modification, transport and degradation of biomolecules as well as signal transduction, which remains challenging to investigate using conventional fluorescent proteins (FPs). Due to the highly acidic luminal environment (pH ~ 4.5⁻6.0), most FPs and related sensors are apt to lose their fluorescence. To address the need to image in acidic environments, several research groups have developed acid-tolerant FPs in a wide color range. Furthermore, the engineering of pH insensitive sensors, and their concomitant use with pH sensitive sensors for the purpose of pH-calibration has enabled characterization of the role of luminal ions. In this short review, we summarize the recent development of acid-tolerant FPs and related functional sensors and discuss the future prospects for this field.

Chemi- and Bioluminescence of Cyclic Peroxides

Bioluminescence is a phenomenon that has fascinated mankind for centuries. Today the phenomenon and its sibling, chemiluminescence, have impacted society with a number of useful applications in fields like analytical chemistry and medicine, just to mention two. In this review, a molecular-orbital perspective is adopted to explain the chemistry behind chemiexcitation in both chemi- and bioluminescence. First, the uncatalyzed thermal dissociation of 1,2-dioxetane is presented and analyzed to explain, for example, the preference for triplet excited product states and increased yield with larger nonreactive substituents. The catalyzed fragmentation reaction and related details are then exemplified with substituted 1,2-dioxetanone species. In particular, the preference for singlet excited product states in that case is explained. The review also examines the diversity of specific solutions both in Nature and in artificial systems and the difficulties in identifying the emitting species and unraveling the color modulation process. The related subject of excited-state chemistry without light absorption is finally discussed. The content of this review should be an inspiration to human design of new molecular systems expressing unique light-emitting properties. An appendix describing the state-of-the-art experimental and theoretical methods used to study the phenomena serves as a complement.

Evolutionary conservation of EF-hand ΙΙ loop in aequorin: Priority of intensity to decay rate in bioluminescence emission

As a Ca²⁺-regulated photoprotein, aequorin (Aeq) contains four EF-hand motifs, the second one lacks the standard sequence for Ca²⁺ coordination and doesn't bind to Ca²⁺. Here, we replaced this loop with a functional loop. According to structural studies, although the global stability of modified aequorin (4EFAeq) is higher than that of Aeq; increasing the local flexibility accompanied by internal structural rearrangements in 4EFAeq result in its penetrability to urea and acrylamide. A fast decay rate was observed for 4EFAeq. Assuming the presence of intermediate states in the luminescent reaction, this observation indicate that the loop replacement leads to the lowering of the half-life of intermediate states which results in increasing the rate of conformational switching of 4EFAeq to light emitting form. However, considerable reduction in initial luminescence intensity of 4EFAeq suggests that the number of functional complexes is reduced. Our findings demonstrate that the conformational effects of the second loop in Aeq elicit a delicate balance between local flexibility and global stability which may be considered as an important functional parameter in photoproteins. It was also concluded that evolutionary conservation of EF-hand ΙΙ in the current form is a consequence of priority of intensity to decay rate in bioluminescent organisms.

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.

Semisynthetic photoprotein reporters for tracking fast Ca(2+) transients

Changes in the intracellular concentration of free ionized calcium ([Ca(2+)]i) control a host of cellular processes as varied as vision, muscle contraction, neuronal signal transmission, proliferation, apoptosis etc. The disturbance in Ca(2+)-signaling causes many severe diseases. To understand the mechanisms underlying the control by calcium and how disorder of this regulation relates to pathological conditions, it is necessary to measure [Ca(2+)]i. The Ca(2+)-regulated photoproteins which are responsible for bioluminescence of marine coelenterates have been successfully used for this purpose over the years. Here we report the results on comparative characterization of bioluminescence properties of aequorin from Aequorea victoria, obelin from Obelia longissima, and clytin from Clytia gregaria charged by native coelenterazine and coelenterazine analogues f, i, and hcp. The comparison of specific bioluminescence activity, stability, emission spectra, stopped-flow kinetics, sensitivity to calcium, and effect of physiological concentrations of Mg(2+) establishes obelin-hcp as an excellent semisynthetic photoprotein to keep track of fast intracellular Ca(2+) transients. The rate of rise of its light signal on a sudden change of [Ca(2+)] is almost 3- and 11-fold higher than those of obelin and aequorin with native coelenterazine, respectively, and 20 times higher than that of the corresponding aequorin-hcp. In addition, obelin-hcp preserves a high specific bioluminescence activity and displays higher Ca(2+)-sensitivity as compared to obelin charged by native coelenterazine and sensitivity to Ca(2+) comparable with those of aequorin-f and aequorin-hcp.

Red-Shifted Aequorin Variants Incorporating Non-Canonical Amino Acids: Applications in In Vivo Imaging

The increased importance of in vivo diagnostics has posed new demands for imaging technologies. In that regard, there is a need for imaging molecules capable of expanding the applications of current state-of-the-art imaging in vivo diagnostics. To that end, there is a desire for new reporter molecules capable of providing strong signals, are non-toxic, and can be tailored to diagnose or monitor the progression of a number of diseases. Aequorin is a non-toxic photoprotein that can be used as a sensitive marker for bioluminescence in vivo imaging. The sensitivity of aequorin is due to the fact that bioluminescence is a rare phenomenon in nature and, therefore, it does not suffer from autofluorescence, which contributes to background emission. Emission of bioluminescence in the blue-region of the spectrum by aequorin only occurs when calcium, and its luciferin coelenterazine, are bound to the protein and trigger a biochemical reaction that results in light generation. It is this reaction that endows aequorin with unique characteristics, making it ideally suited for a number of applications in bioanalysis and imaging. Herein we report the site-specific incorporation of non-canonical or non-natural amino acids and several coelenterazine analogues, resulting in a catalog of 72 cysteine-free, aequorin variants which expand the potential applications of these photoproteins by providing several red-shifted mutants better suited to use in vivo. In vivo studies in mouse models using the transparent tissue of the eye confirmed the activity of the aequorin variants incorporating L-4-iodophehylalanine and L-4-methoxyphenylalanine after injection into the eye and topical addition of coelenterazine. The signal also remained localized within the eye. This is the first time that aequorin variants incorporating non-canonical amino acids have shown to be active in vivo and useful as reporters in bioluminescence imaging.

Improving the luminescence properties of aequorin by conjugating to CdSe/ZnS quantum dot nanoparticles: Red shift and slowing decay rate

Changing the properties of photoprotein aequorin such as the wavelength emission and decay half-life by using bioluminescence resonance energy transfer (BRET) phenomenon is the main aim in this paper. BRET system was set up with CdSe/ZnS quantum dot nanoparticles as an acceptor molecule and photoprotein as an energy donor molecule. Quantum dots are semiconductor nanoparticles with very interesting optical properties, including broad excitation spectra, narrow and the symmetric band width emission spectra, tunable by their sizes, compositions, negligible photo-bleaching and good chemical and photo-stability. In this QD-BRET system, aequorin is conjugated to the carboxyl groups on quantum dot surface by EDC/NHS chemistry as cross linker. Bioluminescence energy generates by aequorin upon adding Ca(2+) and transfers to the quantum dots in a radiationless manner and emits at a longer wavelength. The determined bioluminescent parameters for this method included aequorin activity, emission spectra and decay half-life time. In fact, this spectrum tuning strategy resulted in a change in bioluminescent properties of photoprotein, therefore, the maximum emission wavelength shifted from 455 to 540nm and the decay time increased from 3.76 to 12.11s. Nowadays, photoproteins with different characteristics are capable of being employed as a reporter in multi-analyte detections and in vivo imaging.

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.

Hyperactive Arg39Lys mutated mnemiopsin: implication of positively charged residue in chromophore binding cavity

Mnemiopsin, a Ca²⁺-regulated photoprotein isolated fromMnemiopsis leidyi, belongs to the family of ctenophore photoproteins. While there are no charged amino acid residues in the coelenterazine binding cavity of cnidarian photoproteins, ctenophore photoproteins have a positively charged residue (Arg) in this region.

Aequorin mutants with increased thermostability

Bioluminescent labels can be especially useful for in vivo and live animal studies due to the negligible bioluminescence background in cells and most animals, and the non-toxicity of bioluminescent reporter systems. Significant thermal stability of bioluminescent labels is essential, however, due to the longitudinal nature and physiological temperature conditions of many bioluminescent-based studies. To improve the thermostability of the bioluminescent protein aequorin, we employed random and rational mutagenesis strategies to create two thermostable double mutants, S32T/E156V and M36I/E146K, and a particularly thermostable quadruple mutant, S32T/E156V/Q168R/L170I. The double aequorin mutants, S32T/E156V and M36I/E146K, retained 4 and 2.75 times more of their initial bioluminescence activity than wild-type aequorin during thermostability studies at 37 °C. Moreover, the quadruple aequorin mutant, S32T/E156V/Q168R/L170I, exhibited more thermostability at a variety of temperatures than either double mutant alone, producing the most thermostable aequorin mutant identified thus far.

Genetically encoded Ca2+ indicators in cardiac myocytes

Genetically encoded Ca(2+) indicators constitute a powerful set of tools to investigate functional aspects of Ca(2+) signaling in isolated cardiomyocytes, cardiac tissue, and whole hearts. Here, we provide an overview of the concepts, experiences, state of the art, and ongoing developments in the use of genetically encoded Ca(2+) indicators for cardiac cells and heart tissue. This review is supplemented with in vivo viral gene transfer experiments and comparisons of available genetically encoded Ca(2+) indicators with each other and with the small molecule dye Fura-2. In the context of cardiac myocytes, we provide guidelines for selecting a genetically encoded Ca(2+) indicator. For future developments, we discuss improvements of a broad range of properties, including photophysical properties such as spectral spread and biocompatibility, as well as cellular and in vivo applications.

Neuronal network imaging in acute slices using Ca2+ sensitive bioluminescent reporter

Genetically encoded indicators are valuable tools to study intracellular signaling cascades in real time using fluorescent or bioluminescent imaging techniques. Imaging of Ca(2+) indicators is widely used to record transient intracellular Ca(2+) increases associated with bioelectrical activity. The natural bioluminescent Ca(2+) sensor aequorin has been historically the first Ca(2+) indicator used to address biological questions. Aequorin imaging offers several advantages over fluorescent reporters: it is virtually devoid of background signal; it does not require light excitation and interferes little with intracellular processes. Genetically encoded sensors such as aequorin are commonly used in dissociated cultured cells; however it becomes more challenging to express them in differentiated intact specimen such as brain tissue. Here we describe a method to express a GFP-aequorin (GA) fusion protein in pyramidal cells of neocortical acute slices using recombinant Sindbis virus. This technique allows expressing GA in several hundreds of neurons on the same slice and to perform the bioluminescence recording of Ca(2+) transients in single neurons or multiple neurons simultaneously.

Bioluminescent and spectroscopic properties of His-Trp-Tyr triad mutants of obelin and aequorin

Ca(2+)-regulated photoproteins are responsible for the bioluminescence of a variety of marine organisms, mostly coelenterates. The photoproteins consist of a single polypeptide chain to which an imidazopyrazinone derivative (2-hydroperoxycoelenterazine) is tightly bound. According to photoprotein spatial structures the side chains of His175, Trp179, and Tyr190 in obelin and His169, Trp173, Tyr184 in aequorin are at distances that allow hydrogen bonding with the peroxide and carbonyl groups of the 2-hydroperoxycoelenterazine ligand. We replaced these amino acids in both photoproteins by residues with different hydrogen bond donor-acceptor capacity. All mutants exhibited luciferase-like bioluminescence activity, hardly present in the wild-type photoproteins, and showed low or no photoprotein activity, except for aeqH169Q (24% of wild-type activity), obeW179Y (23%), obeW179F (67%), obeY190F (14%), and aeqY184F (22%). The results clearly support the supposition made from photoprotein spatial structures that the hydrogen bond network formed by His-Trp-Tyr triad participates in stabilizing the 2-hydroperoxy adduct of coelenterazine. These residues are also essential for the positioning of the 2-hydroperoxycoelenterazine intermediate, light emitting reaction, and for the formation of active photoprotein. In addition, we demonstrate that although the positions of His-Trp-Tyr residues in aequorin and obelin spatial structures are almost identical the substitution effects might be noticeably different.

Genetically encoded Ca(2+) indicators: properties and evaluation

Genetically encoded calcium ion (Ca(2+)) indicators have become very useful and widely used tools for Ca(2+) imaging, not only in cellular models, but also in living organisms. However, the in vivo and in situ characterization of these indicators is tedious and time consuming, and it does not provide information regarding the suitability of an indicator for particular experimental environments. Thus, initial in vitro evaluation of these tools is typically performed to determine their properties. In this review, we examined the properties of dynamic range, affinity, selectivity, and kinetics for Ca(2+) indicators. Commonly used strategies for evaluating these properties are presented. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Aequorin-based genetic approaches to visualize Ca2+ signaling in developing animal systems

BACKGROUND

In recent years, as our understanding of the various roles played by Ca2+ signaling in development and differentiation has expanded, the challenge of imaging Ca2+ dynamics within living cells, tissues, and whole animal systems has been extended to include specific signaling activity in organelles and non-membrane bound sub-cellular domains.

SCOPE OF REVIEW

In this review we outline how recent advances in genetics and molecular biology have contributed to improving and developing current bioluminescence-based Ca2+ imaging techniques. Reporters can now be targeted to specific cell types, or indeed organelles or domains within a particular cell.

MAJOR CONCLUSIONS

These advances have contributed to our current understanding of the specificity and heterogeneity of developmental Ca2+ signaling. The improvement in the spatial resolution that results from specifically targeting a Ca2+ reporter has helped to reveal how a ubiquitous signaling messenger like Ca2+ can regulate coincidental but different signaling events within an individual cell; a Ca2+ signaling paradox that until now has been hard to explain.

GENERAL SIGNIFICANCE

Techniques used to target specific reporters via genetic means will have applications beyond those of the Ca2+ signaling field, and these will, therefore, make a significant contribution in extending our understanding of the signaling networks that regulate animal development. This article is part of a Special Issue entitled Biochemical, biophysical and genetic approaches to intracellular calcium signalling.

Mitochondria: the calcium connection

Calcium handling by mitochondria is a key feature in cell life. It is involved in energy production for cell activity, in buffering and shaping cytosolic calcium rises and also in determining cell fate by triggering or preventing apoptosis. Both mitochondria and the mechanisms involved in the control of calcium homeostasis have been extensively studied, but they still provide researchers with long-standing or even new challenges. Technical improvements in the tools employed for the investigation of calcium dynamics have been-and are still-opening new perspectives in this field, and more prominently for mitochondria. In this review we present a state-of-the-art toolkit for calcium measurements, with major emphasis on the advantages of genetically encoded indicators. These indicators can be efficiently and selectively targeted to specific cellular sub-compartments, allowing previously unavailable high-definition calcium dynamic studies. We also summarize the main features of cellular and, in more detail, mitochondrial calcium handling, especially focusing on the latest breakthroughs in the field, such as the recent direct characterization of the calcium microdomains that occur on the mitochondrial surface upon cellular stimulation. Additionally, we provide a major example of the key role played by calcium in patho-physiology by briefly describing the extensively reported-albeit highly controversial-alterations of calcium homeostasis in Alzheimer's disease, casting lights on the possible alterations in mitochondrial calcium handling in this pathology.

Monitoring neural activity with bioluminescence during natural behavior

Existing techniques for monitoring neural activity in awake, freely behaving vertebrates are invasive and difficult to target to genetically identified neurons. We used bioluminescence to non-invasively monitor the activity of genetically specified neurons in freely behaving zebrafish. Transgenic fish with the Ca(2+)-sensitive photoprotein green fluorescent protein (GFP)-Aequorin in most neurons generated large and fast bioluminescent signals that were related to neural activity, neuroluminescence, which could be recorded continuously for many days. To test the limits of this technique, we specifically targeted GFP-Aequorin to the hypocretin-positive neurons of the hypothalamus. We found that neuroluminescence generated by this group of approximately 20 neurons was associated with periods of increased locomotor activity and identified two classes of neural activity corresponding to distinct swim latencies. Our neuroluminescence assay can report, with high temporal resolution and sensitivity, the activity of small subsets of neurons during unrestrained behavior.

Calcium imaging in single neurons from brain slices using bioluminescent reporters

Responses of three bioluminescent Ca(2+) sensors were studied in vitro and in neurons from brain slices. These sensors consisted of tandem fusions of green fluorescent protein (GFP) with the photoproteins aequorin, obelin, or a mutant aequorin with high Ca(2+) sensitivity. Kinetics of GFP-obelin responses to a saturating Ca(2+) concentration were faster than those of GFP-aequorin at all Mg(2+) concentrations tested, whereas GFP-mutant aequorin responses were the slowest. GFP-photoproteins were efficiently expressed in pyramidal neurons following overnight incubation of acute neocortical slices with recombinant Sindbis viruses. Expression of GFP-photoproteins did not result in conspicuous modification of morphological or electrophysiological properties of layer V pyramidal cells. The three sensors allowed the detection of Ca(2+) transients associated with action potential discharge in single layer V pyramidal neurons. In these neurons, depolarizing steps of increasing amplitude elicited action potential discharge of increasing frequency. Bioluminescent responses of the three sensors were similar in several respects: detection thresholds, an exponential increase with stimulus intensity, photoprotein consumptions, and kinetic properties. These responses, which were markedly slower than kinetics measured in vitro, increased linearly during the action potential discharge and decayed exponentially at the end of the discharge. Onset slopes increased with stimulus intensity, whereas decay kinetics remained constant. Dendritic light emission contributed to whole-field responses, but the spatial resolution of bioluminescence imaging was limited to the soma and proximal apical dendrite. Nonetheless, the high signal-to-background ratio of GFP-photoproteins allowed the detection of Ca(2+) transients associated with 5 action potentials in single neurons upon whole-field bioluminescence recordings.

Aequorin variants with improved bioluminescence properties

The photoprotein aequorin has been widely used as a bioluminescent label in immunoassays, for the determination of calcium concentrations in vivo, and as a reporter in cellular imaging. It is composed of apoaequorin (189 amino acid residues), the imidazopyrazine chromophore coelenterazine and molecular oxygen. The emission characteristics of aequorin can be changed by rational design of the protein to introduce mutations in its structure, as well as by substituting different coelenterazine analogues to yield semi-synthetic aequorins. Variants of aequorin were created by mutating residues His16, Met19, Tyr82, Trp86, Trp108, Phe113 and Tyr132. Forty-two aequorin mutants were prepared and combined with 10 different coelenterazine analogues in a search for proteins with different emission wavelengths, altered decay kinetics and improved stability. This spectral tuning strategy resulted in semi-synthetic photoprotein mutants with significantly altered bioluminescent properties.

Lipid perturbations sensitize osmotic down-shock activated Ca2+ influx, a yeast "deletome" analysis

Osmotic down shock causes an immediate influx of Ca2+ in yeast, likely through a membrane stretch-sensitive channel. To see how this channel is constituted and regulated, we screened the collection of 4,906 yeast gene deletants for major changes in this response by luminomtery. We discovered deletants that responded very strongly to much milder down shocks than wild-type required, but show little changes in up-shock response. Of all the possibilities (general metabolism, ion distribution, cytoskeleton, cell wall, membrane receptors, etc.), most of the over-responders turned out to be deleted of proteins functioning in the biogenesis of phospholipids, sphingolipids, or ergosterol. Other over-responders are annotated to have vesicular transport defects, traceable to lipid defects in some cases. The deletant lacking the de novo synthesis of phosphatidylcholine, opi3delta, is by far the strongest over-responder. opi3 deletion does not cause non-specific leakage but greatly sensitizes the force-sensing Ca2+-influx mechanism. Choline supplementation normalizes the opi3delta response. Thus, the osmotic-pressure induced stretch force apparently controls channel activities through lipids. This unbiased examination of the yeast genome supports the view that forces intrinsic to the bilayer are determined by the geometry of the lipids and these forces, in turn, govern the activities of proteins embedded therein.