Rhod-2 based measurements of intracellular calcium in the perfused mouse heart: cellular and subcellular localization and response to positive inotropy

Utilization of the genetically encoded calcium indicator Salsa6F in cardiac applications

Calcium signaling is a critical process required for cellular mechanisms such as cardiomyocyte contraction. The inability of the cell to properly activate or regulate calcium signaling can lead to contractile dysfunction. In isolated cardiomyocytes, calcium signaling has been primarily studied using calcium fluorescent dyes, however these dyes have limited applicability to whole organs. Here, we crossed the Salsa6f mouse which expresses a genetically encoded ratiometric cytosolic calcium indicator with a cardiomyocyte specific inducible cre to temporally-induce expression and studied cytosolic calcium transients in isolated cardiomyocytes and modified Langendorff heart preparations. Isolated cardiomyocytes expressing Salsa6f or Fluo-4AM loaded were compared. We also crossed the Salsa6f mouse with a floxed Polycystin 2 (PC2) mouse to test the feasibility of using the Salsa6f mouse to measure calcium transients in PC2 heterozygous or homozygous knock out mice. Although there are caveats in the applicability of the Salsa6f mouse, there are clear advantages to using the Salsa6f mouse to measure whole heart calcium signals.

Calcium imaging: a technique to monitor calcium dynamics in biological systems

Calcium ion (Ca2+) is a multifaceted signaling molecule that acts as an important second messenger. During the course of evolution, plants and animals have developed Ca2+ signaling in order to respond against diverse stimuli, to regulate a large number of physiological and developmental pathways. Our understanding of Ca2+ signaling and its components in physiological phenomena ranging from lower to higher organisms, and from single cell to multiple tissues has grown exponentially. The generation of Ca2+ transients or signatures for various stress factor is a well-known mechanism adopted in plant and animal systems. However, the decoding of such remarkable signatures is an uphill task and is always an interesting goal for the scientific community. In the past few decades, studies on the concentration and dynamics of intracellular Ca2+ are significantly increasing and have become a trend in modern biology. The advancement in approaches from Ca2+ binding dyes to in vivo Ca2+ imaging through the use of Ca2+ biosensors to achieve spatio-temporal resolution in micro and milliseconds range, provide us phenomenal opportunities to study live cell Ca2+ imaging or dynamics. Here, we describe the usage, improvement and advancement of Ca2+ based dyes, genetically encoded probes and sensors to achieve extraordinary Ca2+ imaging in plants and animals.

Graphical abstract

Utilization of the genetically encoded calcium indicator Salsa6F in cardiac applications

Calcium signaling is a critical process required for cellular mechanisms such as cardiac contractility. The inability of the cell to properly activate or regulate calcium signaling can lead to contractile dysfunction. In isolated cardiomyocytes, calcium signaling has been primarily studied using calcium fluorescent dyes, however these dyes have limited applicability to whole organs. Here, we crossed the Salsa6f mouse which expresses a genetically encoded ratiometric cytosolic calcium indicator with a cardiomyocyte specific inducible cre to temporally-induce expression and studied cytosolic calcium transients in isolated cardiomyocytes and modified Langendorff heart preparations. Isolated cardiomyocytes expressing Salsa6f or Fluo-4AM loaded were compared. We also crossed the Salsa6f mouse with a floxed Polycystin 2 (PC2) mouse to test the feasibility of using the Salsa6f mouse to measure calcium transients in PC2 heterozygous or homozygous knock out mice. Although there are caveats in the applicability of the Salsa6f mouse, there are clear advantages to using the Salsa6f mouse to measure whole heart calcium signals.

Optical Mapping of Cardiomyocytes in Monolayer Derived from Induced Pluripotent Stem Cells

Optical mapping is a powerful imaging technique widely adopted to measure membrane potential changes and intracellular Ca2+ variations in excitable tissues using voltage-sensitive dyes and Ca2+ indicators, respectively. This powerful tool has rapidly become indispensable in the field of cardiac electrophysiology for studying depolarization wave propagation, estimating the conduction velocity of electrical impulses, and measuring Ca2+ dynamics in cardiac cells and tissues. In addition, mapping these electrophysiological parameters is important for understanding cardiac arrhythmia mechanisms. In this review, we delve into the fundamentals of cardiac optical mapping technology and its applications when applied to hiPSC-derived cardiomyocytes and discuss related advantages and challenges. We also provide a detailed description of the processing and analysis of optical mapping data, which is a crucial step in the study of cardiac diseases and arrhythmia mechanisms for extracting and comparing relevant electrophysiological parameters.

Pharmacological inhibition of translocon is sufficient to alleviate endoplasmic reticulum stress and improve Ca2+ handling and contractile recovery of stunned myocardium

INTRODUCTION

The function of endoplasmic reticulum (ER), a Ca²⁺ storage compartment and site of protein folding, is altered by disruption of intracellular homeostasis. Misfolded proteins accumulated in the ER lead to ER stress (ERS), unfolded protein response (UPR) activation and ER Ca²⁺ loss. Myocardial stunning is a temporary contractile dysfunction, which occurs after brief ischemic periods with minimal or no cell death, being oxidative stress and Ca²⁺ overload potential underlying mechanisms. Myocardial stunning induces ERS response with negatively impact on the post-ischemic mechanical performance through an unknown mechanism.

AIMS

In this study, we explored whether ER Ca²⁺ efflux through the translocon, a major Ca²⁺ leak channel, contributes to Ca²⁺ mishandling and the consequent contractile abnormalities of the stunned myocardium.

METHODS

Mechanical performance, cytosolic Ca²⁺, UPR markers and oxidative state were evaluated in perfused rat/mouse hearts subjected to a brief ischemia followed by reperfusion (I/R) in absence or presence of the translocon inhibitor, emetine (1 μM), comparing its effects with those of the chaperones TUDCA (30 μM) and 4-PBA (3 mM).

RESULTS

Emetine treatment precluded the I/R-induced increase in UPR signaling markers and improved the contractile recovery together with a remarkable attenuation in myocardial stiffness when compared to I/R hearts with no drug. This alleviation of I/R-induced mechanical abnormalities was more effective than that obtained with the chemical chaperones, TUDCA and 4-PBA. Moreover, emetine treatment produced a striking improvement in diastolic Ca²⁺ handling with a partial recovery of the I/R-induced oxidative stress.

CONCLUSION

Blocking ER Ca²⁺ store depletion via translocon suppressed ER stress and improved mechanical performance and diastolic Ca²⁺ handling of stunned myocardium. Modulation of translocon permeability emerges as a therapeutic approach to face dysfunctional consequences of the I/R injury.

Biomedical Applications of Translational Optical Imaging: From Molecules to Humans

Light is a powerful investigational tool in biomedicine, at all levels of structural organization. Its multitude of features (intensity, wavelength, polarization, interference, coherence, timing, non-linear absorption, and even interactions with itself) able to create contrast, and thus images that detail the makeup and functioning of the living state can and should be combined for maximum effect, especially if one seeks simultaneously high spatiotemporal resolution and discrimination ability within a living organism. The resulting high relevance should be directed towards a better understanding, detection of abnormalities, and ultimately cogent, precise, and effective intervention. The new optical methods and their combinations needed to address modern surgery in the operating room of the future, and major diseases such as cancer and neurodegeneration are reviewed here, with emphasis on our own work and highlighting selected applications focusing on quantitation, early detection, treatment assessment, and clinical relevance, and more generally matching the quality of the optical detection approach to the complexity of the disease. This should provide guidance for future advanced theranostics, emphasizing a tighter coupling-spatially and temporally-between detection, diagnosis, and treatment, in the hope that technologic sophistication such as that of a Mars rover can be translationally deployed in the clinic, for saving and improving lives.

Exendin-4 stimulates autophagy in pancreatic β-cells via the RAPGEF/EPAC-Ca2+-PPP3/calcineurin-TFEB axis

Macroautophagy/autophagy is critical for the regulation of pancreatic β-cell mass and its deregulation has been implicated in the pathogenesis of type 2 diabetes (T2D). We have previously shown that treatment of pancreatic β-cells with the GLP1R (glucagon like peptide 1 receptor) agonist exendin-4 stimulates autophagic flux in a setting of chronic nutrient excess. The aim of this study was to identify the underlying pathways contributing to enhanced autophagic flux.

Pancreatic β-cells (INS-1E),mouse and human islets were treated with glucolipotoxic stress (0.5 mM palmitate and 25 mM glucose) in the presence of exendin-4. Consistent with our previous work, exendin-4 stimulated autophagic flux. Using chemical inhibitors and siRNA knockdown, we identified RAPGEF4/EPAC2 (Rap guanine nucleotide exchange factor 4) and downstream calcium signaling to be essential for regulation of autophagic flux by exendin-4. This pathway was independent of AMPK and MTOR signaling. Further analysis identified PPP3/calcineurin and its downstream regulator TFEB (transcription factor EB) as key proteins mediating exendin-4 induced autophagy. Importantly, inhibition of this pathway prevented exendin-4-mediated cell survival and overexpression of TFEB mimicked the cell protective effects of exendin-4 in INS-1E and human islets. Moreover, treatment of db/db mice with exendin-4 for 21 days increased the expression of lysosomal markers within the pancreatic islets. Collectively our data identify the RAPGEF4/EPAC2-calcium-PPP3/calcineurin-TFEB axis as a key mediator of autophagic flux, lysosomal function and cell survival in pancreatic β-cells. Pharmacological modulation of this axis may offer a novel therapeutic target for the treatment of T2D.

Abbreviations: AKT1/protein kinase B: AKT serine/threonine kinase 1; AMPK: 5’ AMP-activated protein kinase; CAMKK: calcium/calmodulin-dependent protein kinase kinase; cAMP: cyclic adenosine monophosphate; CASP3: caspase 3; CREB: cAMP response element-binding protein; CTSD: cathepsin D; Ex4: exendin-4(1-39); GLP-1: glucagon like peptide 1; GLP1R: glucagon like peptide 1 receptor; GLT: glucolipotoxicity; INS: insulin; MTOR: mechanistic target of rapamycin kinase; NFAT: nuclear factor of activated T-cells; PPP3/calcineurin: protein phosphatase 3; PRKA/PKA: protein kinase cAMP activated; RAPGEF3/EPAC1: Rap guanine nucleotide exchange factor 3; RAPGEF4/EPAC2: Rap guanine nucleotide exchange factor 4; SQSTM1/p62: sequestosome 1; T2D: type 2 diabetes; TFEB: transcription factor EB

EDNRB isoform 3 confers Temozolomide resistance in A375 melanoma cells by modulating membrane potential, reactive oxygen species and mitochondrial Ca2

Background

The role of endothelin receptor type B (EDNRB) isoform 3 involved in Temozolomide (TMZ)-induced melanoma cell death has not yet been elucidated.

Methods

The subcellular localization of EDNRB isoform 3 was determined by confocal and immunoblotting assays. Silencing EDNRB isoform 3 was performed by CRISPR/Cas9. Apoptosis was assessed by annexin V/propium iodide staining and caspases 3/7/9 activity. Mitochondrial membrane potential, reactive oxygen species and mitochondrial Ca²⁺ were measured by flow cytometry. Apoptosis protein array was applied.

Results

Confocal and immunoblot analyses indicate mitochondrial localization of EDNRB isoform 3 and the first N-terminal (1–22) amino acids are sufficient for its mitochondrial targeting. EDNRB isoform 3 depleted A375 cells significantly confers chemoresistance with mitochondrial depolarization, reduced reactive oxygen species, enhanced mitochondrial Ca²⁺ uptake and decreased caspase 9 activation. Additionally, apoptosis array shows that lack of EDNRB isoform 3 has relatively lower expression of phosphorylation of p53 at S392 and a slightly higher expression of Paraoxonase 2.

Conclusion

Our findings raise the possibility of targeting EDNRB isoform 3 as a new therapeutic strategy in combination with TMZ for melanoma treatment.

The mitochondrial Na(+)/Ca(2+) exchanger may reduce high glucose-induced oxidative stress and nucleotide-binding oligomerization domain receptor 3 inflammasome activation in endothelial cells

Background

The mitochondrial Na⁺/Ca²⁺ exchanger, NCLX, plays an important role in the balance between Ca²⁺ influx and efflux across the mitochondrial inner membrane in endothelial cells. Mitochondrial metabolism is likely to be affected by the activity of NCLX because Ca²⁺ activates several enzymes of the Krebs cycle. It is currently believed that mitochondria are not only centers of energy production but are also important sites of reactive oxygen species (ROS) generation and nucleotide-binding oligomerization domain receptor 3 (NLRP3) inflammasome activation.

Methods & Results

This study focused on NCLX function, in rat aortic endothelial cells (RAECs), induced by glucose. First, we detected an increase in NCLX expression in the endothelia of rats with diabetes mellitus, which was induced by an injection of streptozotocin. Next, colocalization of NCLX expression and mitochondria was detected using confocal analysis. Suppression of NCLX expression, using an siRNA construct (siNCLX), enhanced mitochondrial Ca²⁺ influx and blocked efflux induced by glucose. Unexpectedly, silencing of NCLX expression induced increased ROS generation and NLRP3 inflammasome activation.

Conclusions

These findings suggest that NCLX affects glucose-dependent mitochondrial Ca²⁺ signaling, thereby regulating ROS generation and NLRP3 inflammasome activation in high glucose conditions. In the early stages of high glucose stimulation, NCLX expression increases to compensate in order to self-protect mitochondrial maintenance, stability, and function in endothelial cells.

A century of optocardiography

In the past decade, optical mapping provided crucial mechanistic insight into electromechanical function and the mechanism of ventricular fibrillation. Therefore, to date, optical mapping dominates experimental cardiac electrophysiology. The first cardiac measurements involving optics were done in the early 1900s using the fast cinematograph that later evolved into methods for high-resolution activation and repolarization mapping and stimulation of specific cardiac cell types. The field of "optocardiography," therefore, emerged as the use of light for recording or interfering with cardiac physiology. In this review, we discuss how optocardiography developed into the dominant research technique in experimental cardiology. Furthermore, we envision how optocardiographic methods can be used in clinical cardiology.

Ratiometric imaging of calcium during ischemia-reperfusion injury in isolated mouse hearts using Fura-2

Background

We present an easily implementable method for measuring Fura-2 fluorescence from isolated mouse hearts using a commercially available switching light source and CCD camera. After calibration, it provides a good estimate of intracellular [Ca²⁺] with both high spatial and temporal resolutions, permitting study of changes in dispersion of diastolic [Ca²⁺], Ca²⁺ transient dynamics, and conduction velocities in mouse hearts. In a proof-of-principle study, we imaged isolated Langendorff-perfused mouse hearts with reversible regional myocardial infarctions.

Methods

Isolated mouse hearts were perfused in the Landendorff-mode and loaded with Fura-2. Hearts were then paced rapidly and subjected to 15 minutes of regional ischemia by ligation of the left anterior descending coronary artery, following which the ligation was removed to allow reperfusion for 15 minutes. Fura-2 fluorescence was recorded at regular intervals using a high-speed CCD camera. The two wavelengths of excitation light were interleaved at a rate of 1 KHz with a computer controlled switching light source to illuminate the heart.

Results

Fura-2 produced consistent Ca²⁺ transients from different hearts. Ligating the coronary artery rapidly generated a well defined region with a dramatic rise in diastolic Ca²⁺ without a significant change in transient amplitude; Ca²⁺ handling normalized during reperfusion. Conduction velocity was reduced by around 50% during ischemia, and did not recover significantly when monitored for 15 minutes following reperfusion.

Conclusions

Our method of imaging Fura-2 from isolated whole hearts is capable of detecting pathological changes in intracellular Ca²⁺ levels in cardiac tissue. The persistent change in the conduction velocities indicates that changes to tissue connectivity rather than altered intracellular Ca²⁺ handling may be underlying the electrical instabilities commonly seen in patients following a myocardial infarction.

Hyperuricemia induces endothelial dysfunction via mitochondrial Na+/Ca2+ exchanger-mediated mitochondrial calcium overload

BACKGROUND

Uric acid (UA) has proven to be a causal agent in endothelial dysfunction in which ROS production plays an important role. Calcium overload in mitochondria can promote the mitochondrial production of ROS. We hypothesize that calcium transduction in mitochondria contributes to UA-induced endothelial dysfunction.

METHODS AND RESULTS

We first demonstrated that high concentrations of UA cause endothelial dysfunction, marked by a reduction in eNOS protein expression and NO release in vitro. We further found that a high concentration of UA increased levels of [Ca2+]mito, total intracellular ROS, H2O2, and mitochondrial O2·-, and Δψmito but not the [Ca2+]cyt level. When the mitochondrial calcium channels NCXmito and MCU were blocked by CGP-37157 and Ru360, respectively, the UA-induced increases in the levels of [Ca2+]mito and total intracellular ROS were significantly reduced. Mitochondrial levels of O2·- and Δψmito were reduced by inhibition of NCXmito but not of MCU. Moreover, inhibition of NCXmito, but not of MCU, blocked the UA-induced reductions in eNOS protein expression and NO release.

CONCLUSIONS

The increased generation of mitochondrial O2·- induced by a high concentration of UA is triggered by mitochondrial calcium overload and ultimately leads to endothelial dysfunction. In this process, the activation of NCXmito is the major cause of the influx of calcium into mitochondria. Our results provide a new pathophysiological mechanism for UA-induced endothelial dysfunction and may offer a new therapeutic target for clinicians.

A fast black-blood sequence for four-dimensional cardiac manganese-enhanced MRI in mouse

The increasing number of mouse models of cardiac diseases requires improvements in the current MRI tools. Anatomic and functional cardiac phenotyping by MRI calls for both time and space resolution in three dimensions. Black-blood contrast is often needed for the accurate delineation of myocardium and chambers, and is consistent with manganese contrast enhancement. In this article, we propose a fast, three-dimensional, time-resolved (four-dimensional), black-blood MRI sequence that allows mouse heart imaging at 10 periods of the cardiac cycle within 30  min at an isotropic resolution of 200  µm. Two-dimensional imaging was possible within 80  s. Blood cancellation was achieved by employing bipolar gradients without the use of a double inversion recovery preparation scheme. Saturation slices were added in two-dimensional experiments for better blood nulling. The rapidity of the two-dimensional acquisition protocol allowed the measurement of the time course of contrast enhancement on manganese infusion. Owing to the very high contrast-to-noise ratio, manganese-enhanced MRI in four dimensions made possible the accurate assessment of regional cardiac volumes in healthy animals. In experimentally infarcted mice, the size of the ischemic zone could be measured easily with this method. The technique might be valuable in evaluating mouse heart diseases and their follow-up in longitudinal studies.

Optical mapping of myocardial reactive oxygen species production throughout the reperfusion of global ischemia

Reactive oxygen species (ROS) are short-lived, highly reactive chemical entities that play significant roles in all levels of biology. However, their measurement requires destructive preparation, thereby limiting the continuous measurement of ROS in a living tissue. We develop an optical mapping system to visualize ROS production in an isolated and perfused rat heart. By staining the heart with dihydroethidium (DHE), a 532-nm laser beam is directed to the epicardial surface, where we collect the red fluorescence (>600 nm) for semiquantitative analysis. With this system, ROS production as well as ventricular pressure and ECG in isolated perfused rat hearts are monitored throughout the reperfusion of global ischemia. Ischemia would decrease myocardial ROS production, while reperfusion would immediately result in sustained ROS overproduction. Optical mapping would provide information regarding the spatial distribution and temporal evolution of myocardial ROS production, which would enhance knowledge of the role of free radicals in cardiovascular biology.

Simultaneous detection of blood volume, oxygenation, and intracellular calcium changes during cerebral ischemia and reperfusion in vivo using diffuse reflectance and fluorescence

We describe an approach to measure changes in intracellular calcium along with changes in blood volume and oxygenation directly from the exposed rat cortex in vivo during cerebral ischemia and reperfusion. Measurements were made using a catheter-based optical system. The endface of a Y-shaped bifurcated fiber optic bundle was mounted on the cortical surface. It delivered the light at three wavelengths of 548, 555, and 572 nm to the brain through a fast monochromator coupled to a xenon lamp, and collected the calcium-dependent fluorescence emission from Rhod2 at 589 nm (excited at 548 nm) along with the diffuse reflections at the wavelengths of 555 and 572 nm to determine the changes in blood volume and hemoglobin oxygenation. The feasibility of this approach was experimentally examined by inducing transient cerebral ischemia and reperfusion in the rat. The ischemia induced an 8.5%+/-1.7% fluorescence increase compared with the preischemic control values. Blood volume and tissue hemoglobin oxygenation decreased by 57.4%+/-12.6% and 47.3%+/-12.5%, respectively. All signals normalized on reperfusion. The ischemia-induced change in Rhod2-Ca2+ fluorescence was blocked using a calcium channel blocker, nimodipine, confirming that intracellular changes in calcium were responsible for the fluorescence changes. Thus, changes in cerebral hemodynamics and intracellular calcium concentration changes were measured simultaneously, facilitating future studies of the interrelationship between neuronal activation and metabolic and vascular processes in normal and diseased brain.

Optical imaging of the heart

Optical techniques have revolutionized the investigation of cardiac cellular physiology and advanced our understanding of basic mechanisms of electrical activity, calcium homeostasis, and metabolism. Although optical methods are widely accepted and have been at the forefront of scientific discoveries, they have been primarily applied at cellular and subcellular levels and considerably less to whole heart organ physiology. Numerous technical difficulties had to be overcome to dynamically map physiological processes in intact hearts by optical methods. Problems of contraction artifacts, cellular heterogeneities, spatial and temporal resolution, limitations of surface images, depth-of-field, and need for large fields of view (ranging from 2x2 mm2 to 3x3 cm2) have all led to the development of new devices and optical probes to monitor physiological parameters in intact hearts. This review aims to provide a critical overview of current approaches, their contributions to the field of cardiac electrophysiology, and future directions of various optical imaging modalities as applied to cardiac physiology at organ and tissue levels.

Decreasing motion artifacts in calcium-dependent fluorescence transients from the perfused mouse heart using frequency filtering

A strategy has been developed for the removal of motion artifact and noise in calcium-dependent fluorescence transients from the perfused mouse heart using frequency filtering. An analytical model indicates that the spectral removal of motion artifacts is independent of the phase shift of the motion waveform in the frequency domain, and thus to the time shift (or delay) of motion in the time domain. This is based on the "shift theorem" of Fourier analysis, which avoids erroneous correction of motion artifact when using the motion signal obtained using reflectance from the heart. Several major steps are adopted to implement this model for elimination of motion as well as detection noise from the fluorescence transient signals from the calcium-sensitive probe Rhod-2. These include (1) extracting the fluorescence calcium transient signal from the raw data by using power spectrum density (PSD) in the frequency domain by subtracting the motion recorded using the reflectance of excitation light, (2) digitally filtering out the random noise using multiple bandpass filters centralized at harmonic frequencies of the transients, and (3) extracting high frequency noise with a Gaussian Kernel filter method. The processed signal of transients acquired with excessive motion artifact is comparable to transients acquired with minimal motion obtained by immobilizing the heart against the detection window, demonstrating the usefulness of this technique.

Hydrolysis of Ca2+-sensitive fluorescent probes by perfused rat heart

Rat hearts were loaded with the fluorescent calcium indicators fura 2, indo 1, rhod 2, or fluo 3 to determine cytosolic calcium levels in the perfused rat heart. With fura 2, however, basal tissue fluorescence increased above anticipated levels, suggesting accumulation of intermediates of fura 2-AM deesterification. To examine this process, we separated the intermediates of the deesterification process using HPLC after incubation of fura 2-AM with tissue homogenates and after loading in the rat heart. Loading of hearts with fura 2-AM resulted in tissue levels of fura 2 free acid that were only 5% of the total heart dye content of all fura 2 species. The parent fura 2-AM form accumulated without accumulation of intermediate products. Similar results were obtained with indo 1-AM. Fluo 3 loaded very poorly in perfused hearts. Unlike other indictors, rhod 2 rapidly loaded in perfused hearts and was completely converted to the free acid form. To determine the subcellular localization of the free acid form of these indictors, mitochondria from indicator-loaded hearts were assayed for the free acid form. Approximately 75% of the total amount of rhod 2 in hearts could be recovered in isolated mitochondria. Subcellular localization of indo 1 and fura 2 was more evenly distributed between mitochondria and nonmitochondrial compartments. We conclude that measurement of calcium in the perfused rat heart using surface fluorescence with either indo 1 or fura 2 is complicated by an inconsistent accumulation of the parent ester and that the resulting signal cannot be easily calibrated using “in situ” methods using the free acid form. Rhod 2 does not display this shortcoming, but like other indicators, it also loads into the mitochondrial matrix.

Defects in ryanodine receptor calcium release in skeletal muscle from post-myocardial infarct rats

Defective calcium (Ca2+) signaling and impaired contractile function have been observed in skeletal muscle secondary to impaired myocardial function. However, the molecular basis for these muscle defects have not been identified. In this study, we evaluated the alterations of the ryanodine-sensitive Ca2+ release channels (RyR1) by analyzing global and local Ca2+ signaling in a rat postmyocardial infarction (PMI) model of myocardial overload. Ca2+ transients, measured with multiphoton imaging in individual fibers within a whole extensor digitorum longus (EDL) muscle, exhibited significantly reduced amplitude and a prolonged time course in PMI. Spatio-temporal properties of spontaneous Ca2+ sparks in fibers isolated from PMI EDL muscles were also significantly altered. In addition, RyR1 from PMI skeletal muscles were PKA-hyperphosphorylated and depleted of the FK506 binding protein (FKBP12). These data show that PMI skeletal muscles exhibit altered local Ca2+ signaling, associated with hyperphosphorylation of RyR1. The observed changes in Ca2+ signaling may contribute to defective excitation-contraction coupling in muscle that can contribute to the reduced exercise capacity in PMI, out of proportion to the degree of cardiac dysfunction.

Two-photon molecular excitation imaging of Ca2+ transients in Langendorff-perfused mouse hearts

The ability to image calcium signals at subcellular levels within the intact depolarizing heart could provide valuable information toward a more integrated understanding of cardiac function. Accordingly, a system combining two-photon excitation with laser-scanning microscopy was developed to monitor electrically evoked [Ca(2+)](i) transients in individual cardiomyocytes within noncontracting Langendorff-perfused mouse hearts. [Ca(2+)](i) transients were recorded at depths </=100 microm from the epicardial surface with the fluorescent indicators rhod-2 or fura-2 in the presence of the excitation-contraction uncoupler cytochalasin D. Evoked [Ca(2+)](i) transients were highly synchronized among neighboring cardiomyocytes. At 1 Hz, the times from 90 to 50% (t(90-50%)) and from 50 to 10% (t(50-10%)) of the peak [Ca(2+)](i) were (means +/- SE) 73 +/- 4 and 126 +/- 10 ms, respectively, and at 2 Hz, 62 +/- 3 and 94 +/- 6 ms (n = 19, P < 0.05 vs. 1 Hz) in rhod-2-loaded cardiomyocytes. [Ca(2+)](i) decay was markedly slower in fura-2-loaded hearts (t(90-50%) at 1 Hz, 128 +/- 9 ms and at 2 Hz, 88 +/- 5 ms; t(50-10%) at 1 Hz, 214 +/- 18 ms and at 2 Hz, 163 +/- 7 ms; n = 19, P < 0.05 vs. rhod-2). Fura-2-induced deceleration of [Ca(2+)](i) decline resulted from increased cytosolic Ca(2+) buffering, because the kinetics of rhod-2 decay resembled those obtained with fura-2 after incorporation of the Ca(2+) chelator BAPTA. Propagating calcium waves and [Ca(2+)](i) amplitude alternans were readily detected in paced hearts. This approach should be of general utility to monitor the consequences of genetic and/or functional heterogeneity in cellular calcium signaling within whole mouse hearts at tissue depths that have been inaccessible to single-photon imaging.

Identification and characterization of a novel secretory granule calcium-binding protein from the early branching eukaryote Giardia lamblia

Giardia lamblia is a flagellate protozoan that infects humans and other mammals and the most frequently isolated intestinal parasite worldwide. Giardia trophozoites undergo essential biological changes to survive outside the intestine of their host by differentiating into infective cysts. Cyst formation, or encystation, is considered one of the most primitive adaptive responses developed by eukaryotes early in evolution and crucial for the transmission of the parasite among susceptible hosts. During this process, proteins that will assemble into the extracellular cyst wall (CWP1 and CWP2) are transported to the cell surface within encystation-specific secretory vesicles (ESVs) by a developmentally regulated secretory pathway. Cyst wall proteins (CWPs) are maintained as a dense material inside the ESVs, but after exocytosis, they form the fibrillar matrix of the cyst wall. Little is known about the molecular mechanisms involved in granule biogenesis and discharge in Giardia, as well as the assembly of the extracellular wall. In this work, we provide evidences that a novel 54-kDa protein that exclusively localizes to the ESVs is induced during encystation similar to CWPs, proteolytically processed during granule maturation, and able to bind calcium in vitro. The gene encoding this molecule predicts a novel protein (called gGSP for G. lamblia Granule-specific Protein) without homology to any other protein reported in public databases. Nevertheless, it possesses characteristics of calcium-sequestering molecules of higher eukaryotes. Inhibition of gGSP expression abolishes cyst wall formation, suggesting that this secretory granule protein regulates Ca(2+)-dependent degranulation of ESVs during cyst wall formation.

Compensatory changes in Ca(2+) and myocardial O(2) consumption in beta-tropomyosin transgenic hearts

Transgenic mice overexpressing beta-tropomyosin have increased myofilament Ca(2+) sensitivity that we hypothesized would result in altered relationships among pressure and heart rates, intracellular Ca(2+), and myocardial O(2) consumption. In perfused hearts from transgenic mice there was a marked negative force-frequency response between 6 and 10 Hz with a 30 +/- 3% reduction in peak-positive first derivative of pressure development over time (dP/dt) compared with 14 +/- 2% in wild-type mice (P < 0.001). At 8 Hz systolic pressures were normal, though peak systolic intracellular Ca(2+) was significantly reduced in transgenic mice versus wild type (726 +/- 61 vs. 936 +/- 67 nM, P < 0.05) indicating an alteration in the pressure-Ca(2+) relationship. Over a wide range of positive and negative inotropic interventions there were normal developed pressures, though marked elevations in myocardial O(2) consumption (15-54%). Because pressures are normal and intracellular Ca(2+) decreased and myocardial O(2) consumption increased, this suggests that these abnormalities are at least in part compensatory mechanisms to the altered myofilament function.

High calcium and dobutamine positive inotropy in the perfused mouse heart: myofilament calcium responsiveness, energetic economy, and effects of protein kinase C inhibition

BACKGROUND

In perfused hearts, high calcium-induced inotropy results in less developed pressure relative to myocardial oxygen consumption compared to the beta-adrenergic agonist dobutamine. Calcium handling is an important determinant of myocardial oxygen consumption. Therefore, we hypothesized that this phenomenon was due to reduced myofilament responsiveness to calcium, related to protein kinase C activation.

RESULTS

Developed pressure was significantly higher with dobutamine compared to high perfusate calcium of 3.5 mM (73 +/- 10 vs 63 +/- 10 mmHg, p < 0.05), though peak systolic intracellular calcium was not significantly different, suggesting reduced myofilament responsiveness to intracellular calcium with high perfusate calcium. The ratio of developed pressure to myocardial oxygen consumption, an index of economy of contraction, was significantly increased with dobutamine compared to high perfusate calcium (1.35 +/- 0.15 vs 1.15 +/- 0.15 mmHg/micromoles/min/g dry wt, p < 0.05), suggesting energetic inefficiency with high perfusate calcium. The specific protein kinase C inhibitor, chelerythrine, significantly attenuated the expected increase in developed pressure when increasing perfusate calcium from 2.5 to 3.5 mM (3.5 mM: 64 +/- 8 vs 3.5 mM + chelerythrine: 55 +/- 5 mmHg, p < 0.05), though had no effects on dobutamine, or lower levels of perfusate calcium (1.5 to 2.5 mM).

CONCLUSIONS

By measuring intracellular calcium, developed pressures and myocardial oxygen consumption in perfused mouse hearts, these results demonstrate that high perfusate calcium positive inotropy compared to dobutamine results in reduced myofilament responsiveness to intracellular calcium, which is associated with energetic inefficiency and evidence of protein kinase C activation.

Calcium measurements in perfused mouse heart: quantitating fluorescence and absorbance of Rhod-2 by application of photon migration theory

Both theoretical and experimental results are presented for the quantitative detection of calcium transients in the perfused mouse heart loaded with the calcium-sensitive fluorescent dye Rhod-2. Analytical models are proposed to calculate both the reflected absorbance and fluorescence spectra detected from the mouse heart. These models allow correlation of the measured spectral intensities with the relative quantity of Rhod-2 in the heart and measurement of the changes in quantum yield of Rhod-2 upon binding calcium in the heart in which multiple scattering effects are predominant. Theoretical modeling and experimental results demonstrate that both reflected absorbance and fluorescence emission are attenuated linearly with Rhod-2 washout. According to this relation, a ratiometric method using fluorescence and absorbance is validated as a measure of the quantum yield of calcium-dependent fluorescence, enabling determination of the dynamics of cytosolic calcium in the perfused mouse heart. The feasibility of this approach is confirmed by experiments quantifying calcium transients in the perfused mouse heart stimulated at 8 Hz. The calculated cytosolic calcium concentrations are 368 +/- 68 nM and 654 +/- 164 nM in diastole and systole, respectively. Spectral distortions induced by tissue scattering and absorption and errors induced by the geometry of the detection optics in the calcium quantification are shown to be eliminated by using the ratio method. Methods to effectively minimize motion-induced artifacts and to monitor the oxygenation status of the whole perfused heart are also discussed.