The use of aequorins to record and visualize Ca(2+) dynamics: from subcellular microdomains to whole organisms

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 Use of Complementary Luminescent and Fluorescent Techniques for Imaging Ca2+ Signaling Events During the Early Development of Zebrafish (Danio rerio)

We have visualized many of the Ca²⁺ signaling events that occur during the early stages of zebrafish development using complementary luminescent and fluorescent imaging techniques. We initially microinject embryos with the luminescent Ca²⁺ reporter, f-holo-aequorin, and using a custom-designed luminescent imaging system, we can obtain pan-embryonic visual information continually for up to the first ~24 h postfertilization (hpf). Once we know approximately when and where to look for these Ca²⁺ signaling events within a complex developing embryo, we then repeat the experiment using a fluorescent Ca²⁺ reporter such as calcium green-1 dextran and use confocal laser scanning microscopy to provide time-lapse series of higher-resolution images. These protocols allow us to identify the specific cell types and even the particular subcellular domain (e.g., nucleus or cytoplasm) generating the Ca²⁺ signal. Here, we outline the techniques we use to precisely microinject f-holo-aequorin or calcium green-1 dextran into embryos without affecting their viability or development. We also describe how to inject specific regions of early embryos in order to load localized embryonic domains with a particular Ca²⁺ reporter. These same techniques can also be used to introduce other membrane-impermeable reagents into embryos, including Ca²⁺ channel antagonists, Ca²⁺ chelators, fluorescent dyes, RNA, and DNA.

Ca2+ release via two-pore channel type 2 (TPC2) is required for slow muscle cell myofibrillogenesis and myotomal patterning in intact zebrafish embryos

We recently demonstrated a critical role for two-pore channel type 2 (TPC2)-mediated Ca²⁺ release during the differentiation of slow (skeletal) muscle cells (SMC) in intact zebrafish embryos, via the introduction of a translational-blocking morpholino antisense oligonucleotide (MO). Here, we extend our study and demonstrate that knockdown of TPC2 with a non-overlapping splice-blocking MO, knockout of TPC2 (via the generation of a tpcn2^dhkz1a mutant line of zebrafish using CRISPR/Cas9 gene-editing), or the pharmacological inhibition of TPC2 action with bafilomycin A1 or trans-ned-19, also lead to a significant attenuation of SMC differentiation, characterized by a disruption of SMC myofibrillogenesis and gross morphological changes in the trunk musculature. When the morphants were injected with tpcn2-mRNA or were treated with IP₃/BM or caffeine (agonists of the inositol 1,4,5-trisphosphate receptor (IP₃R) and ryanodine receptor (RyR), respectively), many aspects of myofibrillogenesis and myotomal patterning (and in the case of the pharmacological treatments, the Ca²⁺ signals generated in the SMCs), were rescued. STED super-resolution microscopy revealed a close physical relationship between clusters of RyR in the terminal cisternae of the sarcoplasmic reticulum (SR), and TPC2 in lysosomes, with a mean estimated separation of ~52-87nm. Our data therefore add to the increasing body of evidence, which indicate that localized Ca²⁺ release via TPC2 might trigger the generation of more global Ca²⁺ release from the SR via Ca²⁺-induced Ca²⁺ release.

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.

Two-Pore Channel 2 activity is required for slow muscle cell-generated Ca(2+) signaling during myogenesis in intact zebrafish

We have recently characterized essential inositol 1,4,5-trisphosphate receptor (IP 3R) and ryanodine receptor (RyR)-mediated Ca(2+) signals generated during the differentiation of slow muscle cells (SMCs) in intact zebrafish embryos. Here, we show that the lysosomal two-pore channel 2 (TPC2) also plays a crucial role in generating, and perhaps triggering, these essential Ca(2+) signals, and thus contributes to the regulation of skeletal muscle myogenesis. We used a transgenic line of zebrafish that expresses the bioluminescent Ca(2+) reporter, aequorin, specifically in skeletal muscle, in conjunction with morpholino (MO)-based and pharmacological inhibition of TPC2, in both intact embryos and isolated SMCs. MO-based knock-down of TPC2 resulted in a dramatic attenuation of the Ca(2+) signals, whereas the introduction of TPCN2-MO and TPCN2 mRNA together partially rescued the Ca(2+) signaling signature. Embryos treated with trans-ned-19 or bafilomycin A1, a specific NAADP receptor inhibitor and vacuolar-type H(+)ATPase inhibitor, respectively, also displayed a similar disruption of SMC Ca(2+) signaling. TPC2 and lysosomes were shown via immunohistochemistry and confocal laser scanning microscopy to be localized in perinuclear and striated cytoplasmic domains of SMCs, coincident with patterns of IP 3R and RyR expression. These data together imply that TPC2-mediated Ca(2+) release from lysosomes acts upstream from RyR- and IP 3R-mediated Ca(2+) release, suggesting that the former might act as a sensitive trigger to initiate the SR-mediated Ca(2+)-induced-Ca(2+)-release essential for SMC myogenesis and function.

Methods to monitor and compare mitochondrial and glycolytic ATP production

ATP is commonly considered as the main energy unit of the cell and participates in a variety of cellular processes. Thus, intracellular ATP concentrations rapidly vary in response to a wide variety of stimuli, including nutrients, hormones, cytotoxic agents, and hypoxia. Such alterations not necessarily affect cytosolic and mitochondrial ATP to similar extents. From an oncological perspective, this is particularly relevant in the course of tumor progression as well as in the response of cancer cells to therapy. In normal cells, mitochondrial oxidative phosphorylation (OXPHOS) is the predominant source of ATP. Conversely, many cancer cells exhibit an increased flux through glycolysis irrespective of oxygen tension. Assessing the relative contribution of glycolysis and OXPHOS to intracellular ATP production is fundamental not only for obtaining further insights into the peculiarities and complexities of oncometabolism but also for developing therapeutic and diagnostic tools. Several techniques have been developed to measure intracellular ATP levels including enzymatic methods based on hexokinase, glucose-6-phosphate dehydrogenase, and firefly luciferase. Here, we summarize conventional methods for measuring intracellular ATP levels and we provide a detailed protocol based on cytosol- and mitochondrion-targeted variants of firefly luciferase to determine the relative contribution of glycolysis and OXPHOS to ATP synthesis.

Imaging Ca(2+) activity in mammalian cells and zebrafish with a novel red-emitting aequorin variant

Ca²⁺ monitoring with aequorin is an established bioluminescence technique, whereby the photoprotein emits blue light when it binds to Ca²⁺. However, aequorin’s blue emission and low quantum yield limit its application for in vivo imaging because blue-green light is greatly attenuated in animal tissues. In earlier work, aequorin was molecularly fused with green, yellow, and red fluorescent proteins, producing an emission shift through bioluminescence resonance energy transfer (BRET). We have previously shown that the chimera tandem dimer Tomato-aequorin (tdTA) emits red light in mammalian cells and across the skin and other tissues of mice [1]. In this work, we varied the configuration of the linker in tdTA to maximize energy transfer. One variant, named Redquorin, improved BRET from aequorin to tdTomato to almost a maximum value, and the emission above 575 nm exceeded 73 % of total counts. By pairing Redquorin with appropriate synthetic coelenterazines, agonist-induced and spontaneous Ca²⁺ oscillations in single HEK-293 cells were imaged. In addition, we also imaged Ca²⁺ transients associated with twitching behavior in developing zebrafish embryos expressing Redquorin during the segmentation period. Furthermore, the emission profile of Redquorin resulted in significant luminescence crossing a blood sample, a highly absorbing tissue. This new tool will facilitate in vivo imaging of Ca²⁺ from deep tissues of animals.

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

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

Imaging long distance propagating calcium signals in intact plant leaves with the BRET-based GFP-aequorin reporter

Calcium (Ca(2+)) is a second messenger involved in many plant signaling processes. Biotic and abiotic stimuli induce Ca(2+) signals within plant cells, which, when decoded, enable these cells to adapt in response to environmental stresses. Multiple examples of Ca(2+) signals from plants containing the fluorescent yellow cameleon sensor (YC) have contributed to the definition of the Ca(2+) signature in some cell types such as root hairs, pollen tubes and guard cells. YC is, however, of limited use in highly autofluorescent plant tissues, in particular mesophyll cells. Alternatively, the bioluminescent reporter aequorin enables Ca(2+) imaging in the whole plant, including mesophyll cells, but this requires specific devices capable of detecting the low amounts of emitted light. Another type of Ca(2+) sensor, referred to as GFP-aequorin (G5A), has been engineered as a chimeric protein, which combines the two photoactive proteins from the jellyfish Aequorea victoria, the green fluorescent protein (GFP) and the bioluminescent protein aequorin. The Ca(2+)-dependent light-emitting property of G5A is based on a bioluminescence resonance energy transfer (BRET) between aequorin and GFP. G5A has been used for over 10 years for enhanced in vivo detection of Ca(2+) signals in animal tissues. Here, we apply G5A in Arabidopsis and show that G5A greatly improves the imaging of Ca(2+) dynamics in intact plants. We describe a simple method to image Ca(2+) signals in autofluorescent leaves of plants with a cooled charge-coupled device (cooled CCD) camera. We present data demonstrating how plants expressing the G5A probe can be powerful tools for imaging of Ca(2+) signals. It is shown that Ca(2+) signals propagating over long distances can be visualized in intact plant leaves and are visible mainly in the veins.

Introduction of aequorin into zebrafish embryos for recording Ca(2+) signaling during the first 48 h of development

Ca(2+) signals, whether transient pulses, propagating waves, or long-duration, steady gradients, are generally considered to play an important role in the pattern-forming events that occur during vertebrate development. One vertebrate that has long been a favorite of embryologists because of its ex utero development and the optical clarity of its embryos is the zebrafish, Danio rerio. Using the bioluminescent Ca(2+) reporter aequorin, distinct Ca(2+) signals have been reported for at least the first 48 h of zebrafish development, with signals becoming progressively more complex as the embryo develops. Here we provide a general introduction to aequorin and its use in monitoring Ca(2+) signals and discuss methods for introducing aequorin into zebrafish embryos.

Microinjecting holo-aequorin into dechorionated and intact zebrafish embryos

The injection of holo-aequorin into embryos at the one-cell stage, along with the use of a simple photomultiplier tube or luminescence imaging system, allows transient localized elevations of free cytosolic Ca(2+) to be recorded and observed during the first 24 h of zebrafish development. The technique for loading dechorionated or intact one-cell stage zebrafish embryos with holo-aequorin is described here.

Application of complementary luminescent and fluorescent imaging techniques to visualize nuclear and cytoplasmic Ca²⁺ signalling during the in vivo differentiation of slow muscle cells in zebrafish embryos under normal and dystrophic conditions

1. Evidence is accumulating for a role for Ca²⁺ signalling in the differentiation and development of embryonic skeletal muscle. 2. Imaging of intact, normally developing transgenic zebrafish that express the protein component of the Ca²⁺-sensitive complex aequorin, specifically in skeletal muscle, show that two distinct periods of spontaneous synchronised Ca²⁺ transients occur in the trunk: one at approximately 17.5-19.5 h post-fertilization (h.p.f.; termed signalling period SP1) and the other after approximately 23 h.p.f. (termed SP2). These periods of intense Ca²⁺ signalling activity are separated by a quiet period. 3. Higher-resolution confocal imaging of embryos loaded with the fluorescent Ca²⁺ reporter calcium green-1 dextran shows that the Ca²⁺ signals are generated almost exclusively in the slow muscle cells, the first muscle cells to differentiate, with distinct nuclear and cytoplasmic components. 4. Here, we show that coincidental with the SP1 Ca²⁺ signals, dystrophin becomes localized to the vertical myoseptae of the myotome. Introduction of a dmd morpholino (dmd-MO) resulted in no dystrophin being expressed in the vertical myoseptae, as well as a disruption of myotome morphology and sarcomere organization. In addition, the Ca²⁺ signalling signatures of dmd-MO-injected embryos or homozygous sapje mutant embryos were abnormal such that the frequency, amplitude and timing of the Ca²⁺ signals were altered compared with controls. 5. Our new data suggest that, in addition to a structural role, dystrophin may function in the regulation of [Ca²⁺](i) during the early stages of slow muscle cell differentiation when the Ca²⁺ signals generated in these cells coincide with the first spontaneous contractions of the trunk.

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.

Necessary role for intracellular Ca2+ transients in initiating the apical-basolateral thinning of enveloping layer cells during the early blastula period of zebrafish development

During the early blastula period of zebrafish embryos, the outermost blastomeres begin to undergo a significant thinning in the apical/basolateral dimension to form the first distinct cellular domain of the embryo, the enveloping layer (EVL). During this shape transformation, only the EVL-precursor cells generate a coincidental series of highly restricted Ca(2+) transients. To investigate the role of these localized Ca(2+) transients in this shape-change process, embryos were treated with a Ca(2+) chelator (5,5'-difluoro BAPTA AM; DFB), or the Ca(2+) ionophore (A23187), to downregulate and upregulate the transients, respectively, while the shape-change of the forming EVL cells was measured. DFB was shown to significantly slow, and A23187 to significantly facilitate the shape change of the forming EVL cells. In addition, to investigate the possible involvement of the phosphoinositide and Wnt/Ca(2+) signaling pathways in the Ca(2+) transient generation and/or shape-change processes, embryos were treated with antagonists (thapsigargin, 2-APB and U73122) or an agonist (Wnt-5A) of these pathways. Wnt-5A upregulated the EVL-restricted Ca(2+) transients and facilitated the change in shape of the EVL cells, while 2-APB downregulated the Ca(2+) transients and significantly slowed the cell shape-change process. Furthermore, thapsigargin and U73122 also both inhibited the EVL cell shape-change. We hypothesize, therefore, that the highly localized and coincidental Ca(2+) transients play a necessary role in initiating the shape-change of the EVL cells.

Visualization of Ca²+ signaling during embryonic skeletal muscle formation in vertebrates

Dynamic changes in cytosolic and nuclear Ca(2+) concentration are reported to play a critical regulatory role in different aspects of skeletal muscle development and differentiation. Here we review our current knowledge of the spatial dynamics of Ca(2+) signals generated during muscle development in mouse, rat, and Xenopus myocytes in culture, in the exposed myotome of dissected Xenopus embryos, and in intact normally developing zebrafish. It is becoming clear that subcellular domains, either membrane-bound or otherwise, may have their own Ca(2+) signaling signatures. Thus, to understand the roles played by myogenic Ca(2+) signaling, we must consider: (1) the triggers and targets within these signaling domains; (2) interdomain signaling, and (3) how these Ca(2+) signals integrate with other signaling networks involved in myogenesis. Imaging techniques that are currently available to provide direct visualization of these Ca(2+) signals are also described.