ERG potassium channels and T-type calcium channels contribute to the pacemaker and atrioventricular conduction in zebrafish larvae

AIM

Bradyarrhythmias result from inhibition of automaticity, prolonged repolarization, or slow conduction in the heart. The ERG channels mediate the repolarizing current IKr in the cardiac action potential, whereas T-type calcium channels (TTCC) are involved in the sinoatrial pacemaker and atrioventricular conduction in mammals. Zebrafish have become a valuable research model for human cardiac electrophysiology and disease. Here, we investigate the contribution of ERG channels and TTCCs to the pacemaker and atrioventricular conduction in zebrafish larvae and determine the mechanisms causing atrioventricular block.

METHODS

Zebrafish larvae expressing ratiometric fluorescent Ca2+ biosensors in the heart were used to measure Ca2+ levels and rhythm in beating hearts in vivo, concurrently with contraction and hemodynamics. The atrioventricular delay (the time between the start of atrial and ventricular Ca2+ transients) was used to measure impulse conduction velocity and distinguished between slow conduction and prolonged refractoriness as the cause of the conduction block.

RESULTS

ERG blockers caused bradycardia and atrioventricular block by prolonging the refractory period in the atrioventricular canal and in working ventricular myocytes. In contrast, inhibition of TTCCs caused bradycardia and second-degree block (Mobitz type I) by slowing atrioventricular conduction. TTCC block did not affect ventricular contractility, despite being highly expressed in cardiomyocytes. Concomitant measurement of Ca2+ levels and ventricular size showed mechano-mechanical coupling: increased preload resulted in a stronger heart contraction in vivo.

CONCLUSION

ERG channels and TTCCs influence the heart rate and atrioventricular conduction in zebrafish larvae. The zebrafish lines expressing Ca2+ biosensors in the heart allow us to investigate physiological feedback mechanisms and complex arrhythmias.

Personalized Use of an Adjustable Movement-Controlled Video Game in Obstetric Brachial Plexus Palsy during Physiotherapy Sessions at School: A Case Report

This case study explores the use of a personalized, adjustable Kinect exergame in 10 physiotherapy sessions for a 10-year-old girl with incomplete right-sided obstetric brachial plexus palsy (OBPP). The aim was to observe the impact on the patient's upper limb mobility that could be achieved through maximization of the player's motivation, possibly due to continuous game parameter adjustments. It had been achieved that the patient was playing 87% of the total gaming time with a personally challenging setting that increased her arm speed from one to four movements. Strength in abduction and flexion were increased by 8 N and 7 N, respectively. Furthermore, the patient showed better muscular balance and an increase of 50% in speed of the Jebsen-Taylor hand function test (JTHFT). The patient reported high levels of motivation, low perception of fatigue, and just slight discomfort. The study found that the use of personalized video games as a complement to conventional physiotherapy can be successful in OBPP patients when the game allows for the adjustment of the difficulty level as a response to personal performance. Predefined difficulty levels and automatic performance analysis can be helpful. Results are promising; however, further research is needed to confirm the results.

Early calcium and cardiac contraction defects in a model of phospholamban R9C mutation in zebrafish

The phospholamban mutation Arg 9 to Cys (R9C) has been found to cause a dilated cardiomyopathy in humans and in transgenic mice, with ventricular dilation and premature death. Emerging evidence suggests that phospholamban R9C is a loss-of-function mutation with dominant negative effect on SERCA2a activity. We imaged calcium and cardiac contraction simultaneously in 3 and 9 days-post-fertilization (dpf) zebrafish larvae expressing plnb^R9C in the heart to unveil the early pathological pathway that triggers the disease. We generated transgenic zebrafish lines expressing phospholamban wild-type (Tg(myl7:plnb^wt)) and phospholamban R9C (Tg(myl7:plnb^R9C)) in the heart of zebrafish. To measure calcium and cardiac contraction in 3 and 9 dpf larvae, Tg(myl7:plnb^wt) and Tg(myl7:plnb^R9C) fish were outcrossed with a transgenic line expressing the ratiometric fluorescent calcium biosensor mCyRFP1-GCaMP6f. We found that Plnb^R9C raised calcium transient amplitude, induced positive inotropy and lusitropy, and blunted the β-adrenergic response to isoproterenol in 3 dpf larvae. These effects can be attributed to enhanced SERCA2a activity induced by the Plnb^R9C mutation. In contrast, Tg(myl7:plnb^R9C) larvae at 9 dpf exhibited ventricular dilation, systolic dysfunction and negative lusitropy, hallmarks of a dilated cardiomyopathy in humans. Importantly, N-acetyl-L-cysteine rescued this deleterious phenotype, suggesting that reactive oxygen species contribute to the pathological pathway. These results also imply that dysregulation of calcium homeostasis during embryo development contributes to the disease progression at later stages. Our in vivo model in zebrafish allows characterization of pathophysiological mechanisms leading to heart disease, and can be used for screening of potential therapeutical agents.

Phenotyping calcium and contraction in a zebrafish model of phospholamban R9C mutation

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Ministry of Science, Innovation and Universities, Spain by Consejería de Educación, Cultura y Deportes. Junta de Comunidades de Castilla-La Mancha.

Background

Phospholamban regulates SERCA2a pump activity by reducing its calcium affinity in the heart. Under β-adrenergic stimulation, protein kinase A (PKA) phosphorylates phospholamban attenuating its inhibitory effect on SERCA2a. The phospholamban mutation Arg 9 to Cys (R9C) was firstly identified in an American family with ventricular dilatation and premature death, and subsequently in a cohort of dilated cardiomyopathy patients in South Africa. Emerging evidence suggests that phospholamban R9C is a loss-of-function mutation with autosomal dominant inheritance. In vitro studies revealed that it enhances pentamer assembly by disulfide bridge formation, reducing the availability of phospholamban monomers to regulate SERCA2a.

Purpose

The present study aimed to investigate defects in Ca2+ handling and cardiac function of phospholamban R9C mutation in an in vivo zebrafish model.

Methods

We generated transgenic zebrafish lines overexpressing phospholamban wild-type (TgPLNwt) and phospholamban R9C (TgPLNR9C) in the heart of zebrafish. To measure calcium kinetics in 3 day-old larvae, TgPLNwt and TgPLNR9C fish were outcrossed with transgenic zebrafish lines expressing the fluorescent calcium biosensor mCyRFP1-GCaMP6f (TgRFP-GcaMP6f) and the bioluminescence calcium biosensor GFP-Aequorin (TgGA) in the heart.

Results

Overexpression of phospholamban wild-type and R9C did not alter the heart rate compared to sibling larvae, and no differences were observed in the upstroke and recovery time of the calcium transients.  However, atrial and ventricular calcium transient amplitude were higher in TgPLNR9C larvae with respect to TgPLNwt larvae. Contractile force of the heart, assessed as the fractional shortening, increased in TgPLNR9C larvae compared to siblings and TgPLNwt. Furthermore, stroke volume and cardiac output were also higher in TgPLNR9C larvae. Bioluminescence experiments were performed to estimate the average calcium levels in the heart. We found that TgPLNR9C larvae showed higher calcium levels than sibling and TgPLNwt larvae. To evaluate the responsiveness to β-adrenergic stimulation, larvae were treated with the β-agonist isoproterenol. Calcium transient amplitude and contractile force increased in sibling and TgPLNwt larvae after treatment. However, this response was absent in TgPLNR9C larvae, suggesting that they were hyper-stimulated in basal conditions, displaying amplitude and contractile force similar to sibling and TgPLNwt larvae treated with isoproterenol.

Conclusion

TgPLNR9C larvae exhibited aberrant calcium handling in the heart with higher calcium levels and contractility, consistent with a lack of inhibition of SERCA2a. Consequently, TgPLNR9C larvae did not respond to β-adrenergic stimulation. These results were consistent with previous in vitro studies carried out in cardiomyocites infected with phospholamban R9C and highlighted the pathological defects of phospholamban R9C at early stages of the disease.

Calcium, contraction, and hemodynamic alterations induced by arrhythmogenic drugs in the zebrafish larvae

Funding Acknowledgements

Type of funding sources: Public grant(s) – EU funding. Main funding source(s): MCIN/AEI/ 10.13039/501100011033 and “ERDF A way of making Europe” by the European Union

Background

Ca2+ ion is pivotal in excitation-contraction coupling. The dysregulation of Ca2+ fluxes is at the core of inherited and acquired arrhythmias. In recent years zebrafish have become an attractive vertebrate model to study heart development, cardiotoxicity, and heart diseases. We recently reported that the transgenic zebrafish line Tg(myl7:Twitch-4) expressing the ratiometric Ca2+ indicator Twitch-4 in the heart, allowed imaging Ca2+ transients (CaT) simultaneously with heart contraction in zebrafish larvae, thus maintaining the physiological mechano-electrical feedback.

Purpose

This study aimed to detect subtle disturbances in Ca2+ cycling, contractility, and hemodynamics induced by arrhythmogenic drugs in zebrafish larvae.

Methods

We have used the zebrafish transgenic line Tg(myl7:Twitch-4) and a custom software to acquire and analyze fluorescence emission ratio images of the heart. Larvae of 3- and 5-days post-fertilization (dpf) were incubated with arrhythmogenic drugs, and their effects on the systolic and diastolic Ca2+ and the CaT amplitude in the atrium and ventricle were determined. As the ventricular diameter was assessed in the same fluorescence images, a simultaneous, real-time, measurement of contraction correlated with Ca2+ was obtained. The systolic and diastolic areas, measured independently, were used to estimate hemodynamic parameters.

Results

Dofetilide caused time-dependent bradycardia and ultimately a 2:1 arrhythmia in 3 and 5 dpf larvae, with an increase in the CaT amplitude. the ejection fraction and the stroke volume increased, but the cardiac output decreased due to the low heart rate (2:1 arrhythmia). The T-type Ca2+ channel antagonist ML218 induced bradycardia and decreased systolic and diastolic Ca2+ levels in 3 dpf larvae, but the CaT amplitude did not change. Interestingly, some larvae (3 out of 14) displayed a progressive lengthening of the time between the start of atrial and ventricular CaT, analogous to increased PR interval of the electrocardiogram, until a ventricular systole failed. This is reminiscent of a second-degree atrioventricular block in humans. At higher ML218 concentration, a 2:1 arrhythmia was observed and increased ventricular CaT amplitude in all larvae.

Conclusions

We characterized Ca2+ changes induced by arrhythmogenic drugs and correlated them with contractility and hemodynamic parameters. The zebrafish line Tg(myl7:Twitch-4) allowed investigating drug-induced arrhythmias in this animal model with unprecedented detail. This transgenic line will serve to screen cardiotoxicity during drug development and to study conduction and rhythm defects in zebrafish, and their correlation with human disease.

Optimized Aequorin Reconstitution Protocol to Visualize Calcium Ion Transients in the Heart of Transgenic Zebrafish Embryos In Vivo

We introduce how to image calcium ion levels in the heart of zebrafish embryos and larvae up to 5 days post-fertilization with the photoprotein green fluorescent protein (GFP)-aequorin (GA) in the transgenic line Tg(myl7:GA). Incubation of the embryos with CTZ to obtain the functional photoprotein yields few emission counts, suggesting that, when the heart is beating, the rate of aequorin consumption is faster than that of the reconstitution with CTZ. In this chapter, we present an improved aequorin reconstitution protocol. We further describe the experimental procedure as well as the bioluminescence data analysis and processing.

Simultaneous imaging of calcium and contraction in the beating heart of zebrafish larvae

In vivo models of cardiac function maintain the complex relationship of cardiomyocytes with other heart cells, as well as the paracrine and mechanoelectrical feedback mechanisms. We aimed at imaging calcium transients simultaneously with heart contraction in zebrafish larvae.

Methods: To image calcium in beating hearts, we generated a zebrafish transgenic line expressing the FRET-based ratiometric biosensor Twitch-4. Since emission ratioing canceled out the motion artifacts, we did not use myosin inhibitors or tnnt2a morpholinos to uncouple contraction from changes in calcium levels. We wrote an analysis program to automatically calculate kinetic parameters of the calcium transients. In addition, the ventricular diameter was determined in the fluorescence images providing a real-time measurement of contraction correlated with calcium.

Results: Expression of Twitch-4 did not affect the force of contraction, the size of the heart nor the heart rate in 3- and 5-days post-fertilization (dpf) larvae. Comparison of 3 and 5 dpf larvae showed that calcium levels and transient amplitude were larger at 5 dpf, but the fractional shortening did not change. To validate the model, we evaluated the effect of drugs with known effects on cardiomyocytes. Calcium levels and the force of contraction decreased by the L-type calcium channel blocker nifedipine, whereas they increased with the activator Bay-K 8644. Caffeine induced bradycardia, markedly decreased ventricular diastolic calcium levels, increased the size of the calcium transients, and caused an escape rhythm in some larvae.

Conclusions: The Tg(myl7:Twitch-4) line provides a physiological approach to image systolic and diastolic calcium levels in the heart of zebrafish larvae. Since the heart is beating, calcium levels and contraction can be correlated. This line will be a useful tool to address pathophysiological mechanisms in diseases like heart failure and arrhythmia, in cardiotoxicity studies and for drug screening.

Cardioluminescence in Transgenic Zebrafish Larvae: A Calcium Imaging Tool to Study Drug Effects and Pathological Modeling

Zebrafish embryos and larvae have emerged as an excellent model in cardiovascular research and are amenable to live imaging with genetically encoded biosensors to study cardiac cell behaviours, including calcium dynamics. To monitor calcium ion levels in three to five days post-fertilization larvae, we have used bioluminescence. We generated a transgenic line expressing GFP-aequorin in the heart, Tg(myl7:GA), and optimized a reconstitution protocol to boost aequorin luminescence. The analogue diacetylh-coelenterazine enhanced light output and signal-to-noise ratio. With this cardioluminescence model, we imaged the time-averaged calcium levels and beat-to-beat calcium oscillations continuously for hours. As a proof-of-concept of the transgenic line, changes in ventricular calcium levels were observed by Bay K8644, an L-type calcium channel activator and with the blocker nifedipine. The β-adrenergic blocker propranolol decreased calcium levels, heart rate, stroke volume, and cardiac output, suggesting that larvae have a basal adrenergic tone. Zebrafish larvae treated with terfenadine for 24 h have been proposed as a model of heart failure. Tg(myl7:GA) larvae treated with terfenadine showed bradycardia, 2:1 atrioventricular block, decreased time-averaged ventricular calcium levels but increased calcium transient amplitude, and reduced cardiac output. As alterations of calcium signalling are involved in the pathogenesis of heart failure and arrhythmia, the GFP-aequorin transgenic line provides a powerful platform for understanding calcium dynamics.

The p.E152K-STIM1 mutation deregulates Ca2+ signaling contributing to chronic pancreatitis

Since deregulation of intracellular Ca²⁺ can lead to intracellular trypsin activation, and stromal interaction molecule-1 (STIM1) protein is the main regulator of Ca²⁺ homeostasis in pancreatic acinar cells, we explored the Ca²⁺ signaling in 37 STIM1 variants found in three pancreatitis patient cohorts. Extensive functional analysis of one particular variant, p.E152K, identified in three patients, provided a plausible link between dysregulated Ca²⁺ signaling within pancreatic acinar cells and chronic pancreatitis susceptibility. Specifically, p.E152K, located within the STIM1 EF-hand and sterile α-motif domain, increased the release of Ca²⁺ from the endoplasmic reticulum in patient-derived fibroblasts and transfected HEK293T cells. This event was mediated by altered STIM1-sarco/endoplasmic reticulum calcium transport ATPase (SERCA) conformational change and enhanced SERCA pump activity leading to increased store-operated Ca²⁺ entry (SOCE). In pancreatic AR42J cells expressing the p.E152K variant, Ca²⁺ signaling perturbations correlated with defects in trypsin activation and secretion, and increased cytotoxicity after cholecystokinin stimulation.This article has an associated First Person interview with the first author of the paper.

Inhibition of the mitotic kinase PLK1 overcomes therapeutic resistance to BET inhibitors in triple negative breast cancer

The inhibition of bromo- and extraterminal domains (BET) has shown an anti-proliferative effect in triple negative breast cancer (TNBC). In this article we explore mechanisms of resistance to BET inhibitors (BETi) in TNBC, with the aim of identifying novel ways to overcome such resistance. Two cellular models of acquired resistance to the BET inhibitor JQ1 were generated using a pulsed treatment strategy. MTT, colony formation, and cytometry assays revealed that BETi-resistant cells were particularly sensitive to PLK1 inhibition. Targeting of the latter reduced cell proliferation, especially in resistant cultures. Quantitative PCR analysis of a panel of mitotic kinases uncovered an increased expression of AURKA, TTK, and PLK1, confirmed by Western blot. Only pharmacological inhibition of PLK1 showed anti-proliferative activity on resistant cells, provoking G2/M arrest, increasing expression levels of cyclin B, pH3 and phosphorylation of Bcl-2 proteins, changes that were accompanied by induction of caspase-dependent apoptosis. JQ1-resistant cells orthotopically xenografted into the mammary fat pad of mice led to tumours that retained JQ1-resistance. Administration of the PLK1 inhibitor volasertib resulted in tumour regression. These findings open avenues to explore the future use of PLK1 inhibitors in the clinical setting of BETi-resistant patients.

Visualization of Mitochondrial Ca2+ Signals in Skeletal Muscle of Zebrafish Embryos with Bioluminescent Indicators

Mitochondria are believed to play an important role in shaping the intracellular Ca²⁺ transients during skeletal muscle contraction. There is discussion about whether mitochondrial matrix Ca²⁺ dynamics always mirror the cytoplasmic changes and whether this happens in vivo in whole organisms. In this study, we characterized cytosolic and mitochondrial Ca²⁺ signals during spontaneous skeletal muscle contractions in zebrafish embryos expressing bioluminescent GFP-aequorin (GA, cytoplasm) and mitoGFP-aequorin (mitoGA, trapped in the mitochondrial matrix). The Ca²⁺ transients measured with GA and mitoGA reflected contractions of the trunk observed by transmitted light. The mitochondrial uncoupler FCCP and the inhibitor of the mitochondrial calcium uniporter (MCU), DS16570511, abolished mitochondrial Ca²⁺ transients whereas they increased the frequency of cytosolic Ca²⁺ transients and muscle contractions, confirming the subcellular localization of mitoGA. Mitochondrial Ca²⁺ dynamics were also determined with mitoGA and were found to follow closely cytoplasmic changes, with a slower decay. Cytoplasmic Ca²⁺ kinetics and propagation along the trunk and tail were characterized with GA and with the genetically encoded fluorescent Ca²⁺ indicator, Twitch-4. Although fluorescence provided a better spatio-temporal resolution, GA was able to resolve the same kinetic parameters while allowing continuous measurements for hours.

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.

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.

Failure of prion protein oxidative folding guides the formation of toxic transmembrane forms

The mechanism by which pathogenic mutations in the globular domain of the cellular prion protein (PrP(C)) increase the likelihood of misfolding and predispose to diseases is not yet known. Differences in the evidences provided by structural and metabolic studies of these mutants suggest that in vivo folding could be playing an essential role in their pathogenesis. To address this role, here we use the single or combined M206S and M213S artificial mutants causing labile folds and express them in cells. We find that these mutants are highly toxic, fold as transmembrane PrP, and lack the intramolecular disulfide bond. When the mutations are placed in a chain with impeded transmembrane PrP formation, toxicity is rescued. These results suggest that oxidative folding impairment, as on aging, can be fundamental for the genesis of intracellular neurotoxic intermediates key in prion neurodegenerations.

Imaging local estrogen production in single living cells with recombinant fluorescent indicators

Estrogens are steroid hormones with many systemic effects in addition to development and maintenance of the female reproductive system, and ligands of estrogen receptors are of clinical importance because of their use as oral contraceptive, hormone replacement and antitumoral therapy. In addition, tumoral tissues have been found to express aromatase and other steroidogenic enzymes synthesizing estradiol. To aid in the understanding of these processes, we have developed assays to image the local production of estrogens in isolated living mammalian cells. We constructed biosensors based on estrogen receptor α ligand binding domain and fluorescent proteins by following two approaches. First, the ligand binding domain and a short fragment of steroid receptor coactivator-1 were appended to a circularly permuted yellow fluorescent protein to construct an excitation ratio estrogen indicator. In the second strategy, we constructed emission ratio sensors based on fluorescence resonance energy transfer, containing the ligand binding domain flanked by donor and acceptor fluorescent proteins. Estrogens altered the fluorescence signal of cells transfected with the indicators in a dose-dependent manner. We imaged local estrogen production in adrenocortical H295 cells expressing aromatase and transfected with the fluorescent sensors. In addition, paracrine detection was observed in HeLa cells harboring the indicators and co-cultured with H295 cells. This imaging approach may allow detection of physiological levels of these hormones in suitable animal models.

Imaging FRET standards by steady-state fluorescence and lifetime methods

Imaging fluorescence resonance energy transfer (FRET) between molecules labeled with fluorescent proteins is emerging as a powerful tool to study changes in ions, ligands, and molecular interactions in their physiological cellular environment. Different methods use either steady-state fluorescence properties or lifetime to quantify the FRET rate. In addition, some provide the absolute FRET efficiency whereas others are simply a relative index very much influenced by the actual settings and instrumentation used, which makes the interpretation of a given FRET rate very difficult. The use and exchange of FRET standards in laboratories using these techniques would help to overcome this drawback. We report here the construction and systematic evaluation of FRET standard probes of varying FRET efficiencies. The standards for intramolecular FRET were protein fusions of the cyan and yellow variants of A. victoria green fluorescent protein (ECFP and citrine) joined by short linkers or larger protein spacers, or ECFP tagged with a tetracysteine motif and labeled with the biarsenical fluorochrome, FlAsH. Negative and positive controls of intermolecular FRET were also used. We compared these FRET standards with up to four FRET quantification methods: ratioing of acceptor to donor emission, donor intensity recovery upon acceptor photobleach, sensitized emission after spectral unmixing of raw images, and fluorescence lifetime imaging (FLIM). The latter was obtained with a frequency-domain setup able to provide high quality lifetime images in less than a second, and is thus very well suited for live cell studies. The FRET rates or indexes of the standards were in good agreement regardless of the method used. For the CFP-tetraCys/FlAsH pair, the rate calculated from CFP quenching was faster than that obtained by FLIM.

Red and green aequorins for simultaneous monitoring of Ca2+ signals from two different organelles

Simultaneous control of different functions by calcium signals is possible because of subcellular compartmentalization. Targeted chemiluminescent Ca2+ probes, such as aequorins (AEQs) are optimal for detecting signals originating in different subcellular domains, but imaging is difficult because of low photon yield causing poor spatiotemporal resolution. To overcome this problem, we have co-expressed two spectrally distinct AEQs in different subcellular locations within the same cells. Seven chimeric proteins containing either green- or red-emitting AEQs, with different targeting sequences and Ca2+ affinities, have been designed and tested. We show here evidence for physical and functional independence of the different probes. Cytosolic Ca2+ signals were mirrored in the nucleus, but amplified inside mitochondria and endoplasmic reticulum, and had different time courses in the various locations. Our results demonstrate that these novel tools permit simultaneous and independent monitoring of [Ca2+] in different subcellular domains of the same cell.

Sprouty2 binds Grb2 at two different proline-rich regions, and the mechanism of ERK inhibition is independent of this interaction

Sprouty2 has been widely implicated in the negative regulation of the fibroblast growth factor receptor-extracellular regulated kinase (ERK) pathway. Sprouty2 directly interacts with the adapter protein Grb2, member of the receptor tyrosine kinase-induced signaling pathways. In considering the functional role of Grb2, we investigated whether the interaction with this protein was responsible for ERK pathway inhibition. We found that the binding between Sprouty2 and Grb2 is constitutive, independent of Sprouty2 tyrosine phosphorylation, although it is increased when fibroblast growth factor receptor is activated. This connection is mediated by the N-terminal SH3 domain of Grb2 and two Sprouty2 proline-rich stretches (residues 59-64 and 303-307). Most importantly, a double Sprouty2 mutant (hSpry2 P59AP304A), which is unable to bind Grb2, developed at a similar inhibition level of fibroblast growth factor receptor-ERK pathway than that which originated from Sprouty2 wt. These results are evidence that the Sprouty2 mechanism of ERK inhibition is independent of Grb2 binding.

Glitazones Differentially Regulate Primary Astrocyte and Glioma Cell Survival

The glitazones or thiazolidinediones are ligands of the peroxisome proliferator-activated receptor gamma (PPARgamma). The glitazones are used in the treatment of diabetes, regulate adipogenesis, inflammation, cell proliferation, and induce apoptosis in several cancer cell types. High grade astrocytomas are rapidly growing tumors derived from astrocytes, for which new treatments are needed. We determined the effects of two glitazones, ciglitazone and the therapeutic rosiglitazone, on the survival of serum-deprived primary rat astrocytes and glioma cell lines C6 and U251, which were assessed by the methylthiazolyl tetrazolium assay and lactate dehydrogenase release. Rosiglitazone (5-20 microM) decreased survival of glioma cells without affecting primary astrocytes, whereas ciglitazone at 20 microM was toxic for both cell types. Ciglitazone at 10 microM was cytoprotective for primary astrocytes but toxic to glioma cells. Cell death induced by ciglitazone, but not rosiglitazone, presented apoptotic features (Hoechst staining and externalization of phosphatidylserine). Two mechanisms to explain cytotoxicity were investigated: activation of PPARgamma and production of reactive oxygen species (ROS). PPARgamma does not seem to be the main mechanism involved, because the order of efficacy for cytotoxicity, ciglitazone > rosiglitazone, was inverse of their reported affinities for activating PPARgamma. In addition, GW9662, an inhibitor of PPARgamma, only slightly attenuated cytotoxicity. However, the rapid increase in ROS production and the marked reduction of cell death with the antioxidants ebselen and N-acetylcysteine, indicate that ROS have a key role in glitazone cytotoxicity. Ciglitazone caused a dose-dependent and rapid loss (in minutes) of mitochondrial membrane potential in glioma cells. Therefore, mitochondria are a likely source of ROS and early targets of glitazone cytotoxicity. Our results highlight the potential of rosiglitazone and related compounds for the treatment of astrogliomas.