Intracellular free Ca2+ movements in cultured cardiac myocytes as shown by rapid scanning confocal microscopy

Nuclear Calcium in Cardiac (Patho)Physiology: Small Compartment, Big Impact

The nucleus of a cardiomyocyte has been increasingly recognized as a morphologically distinct and partially independent calcium (Ca²⁺) signaling microdomain, with its own Ca²⁺-regulatory mechanisms and important effects on cardiac gene expression. In this review, we (1) provide a comprehensive overview of the current state of research on the dynamics and regulation of nuclear Ca²⁺ signaling in cardiomyocytes, (2) address the role of nuclear Ca²⁺ in the development and progression of cardiac pathologies, such as heart failure and atrial fibrillation, and (3) discuss novel aspects of experimental methods to investigate nuclear Ca²⁺ handling and its downstream effects in the heart. Finally, we highlight current challenges and limitations and recommend future directions for addressing key open questions.

Calcium Signaling and Transcriptional Regulation in Cardiomyocytes

Calcium (Ca 2+ ) is a universal regulator of various cellular functions. In cardiomyocytes, Ca 2+ is the central element of excitation–contraction coupling, but also impacts diverse signaling cascades and influences the regulation of gene expression, referred to as excitation–transcription coupling. Disturbances in cellular Ca 2+ -handling and alterations in Ca 2+ -dependent gene expression patterns are pivotal characteristics of failing cardiomyocytes, with several excitation–transcription coupling pathways shown to be critically involved in structural and functional remodeling processes. Thus, targeting Ca 2+ -dependent transcriptional pathways might offer broad therapeutic potential. In this article, we (1) review cytosolic and nuclear Ca 2+ dynamics in cardiomyocytes with respect to their impact on Ca 2+ -dependent signaling, (2) give an overview on Ca 2+ -dependent transcriptional pathways in cardiomyocytes, and (3) discuss implications of excitation–transcription coupling in the diseased heart.

Calsequestrin accumulation in rough endoplasmic reticulum promotes perinuclear Ca2+ release

Molecular mechanisms underlying Ca(2+) regulation by perinuclear endoplasmic/sarcoplasmic reticulum (ER/SR) cisternae in cardiomyocytes remain obscure. To investigate the mechanisms of changes in cardiac calsequestrin (CSQ2) trafficking on perinuclear Ca(2+) signaling, we manipulated the subcellular distribution of CSQ2 by overexpression of CSQ2-DsRed, which specifically accumulates in the perinuclear rough ER. Adult ventricular myocytes were infected with adenoviruses expressing CSQ2-DsRed, CSQ2-WT, or empty vector. We found that perinuclear enriched CSQ2-DsRed, but not normally distributed CSQ2-WT, enhanced nuclear Ca(2+) transients more potently than cytosolic Ca(2+) transients. Overexpression of CSQ2-DsRed produced more actively propagating Ca(2+) waves from perinuclear regions than did CSQ2-WT. Activities of the SR/ER Ca(2+)-ATPase and ryanodine receptor type 2, but not inositol 1,4,5-trisphosphate receptor type 2, were required for the generation of these perinuclear initiated Ca(2+) waves. In addition, CSQ2-DsRed was more potent than CSQ2-WT in inducing cellular hypertrophy in cultured neonatal cardiomyocytes. Our data demonstrate for the first time that CSQ2 retention in the rough ER/perinuclear region promotes perinuclear Ca(2+) signaling and predisposes to ryanodine receptor type 2-mediated Ca(2+) waves from CSQ2-enriched perinuclear compartments and myocyte hypotrophy. These findings provide new insights into the mechanism of CSQ2 in Ca(2+) homeostasis, suggesting that rough ER-localized Ca(2+) stores can operate independently in raising levels of cytosolic/nucleoplasmic Ca(2+) as a source of Ca(2+) for Ca(2+)-dependent signaling in health and disease.

Structural evidence for perinuclear calcium microdomains in cardiac myocytes

At each heartbeat, cardiac myocytes are activated by a cytoplasmic Ca(2+) transient in great part due to Ca(2+) release from the sarcoplasmic reticulum via ryanodine receptors (RyRs) clustered within calcium release units (peripheral couplings/dyads). A Ca(2+) transient also occurs in the nucleoplasm, following the cytoplasmic transient with some delay. Under conditions where the InsP3 production is stimulated, these Ca(2+) transients are regulated actively, presumably by an additional release of Ca(2+) via InsP3 receptors (InsP3Rs). This raises the question whether InsP3Rs are appropriately located for this effect and whether sources of InsP3 and Ca(2+) are available for their activation. We have defined the structural basis for InsP3R activity at the nucleus, using immunolabeling for confocal microscopy and freeze-drying/shadowing, T tubule "staining" and thin sectioning for electron microscopy. By these means we establish the presence of InsP3R at the outer nuclear envelope and show a close spatial relationship between the nuclear envelope, T tubules (a likely source of InsP3) and dyads (the known source of Ca(2+)). The frequency, distribution and distance from the nucleus of T tubules and dyads appropriately establish local perinuclear Ca(2+) microdomains in cardiac myocytes.

Effects of aconitine on [Ca2+] oscillation in cultured myocytes of neonatal rats

In order to investigate the effects of aconitine on [Ca2+] oscillation patterns in cultured myocytes of neonatal rats, fluorescent Ca2+ indicator Fluo-4 NW and laser scanning confocal microscope (LSCM) were used to detect the real-time changes of [Ca2+] oscillation patterns in the cultured myocytes before and after aconitine (1.0 micromol/L) incubation or antiarrhythmic peptide (AAP) and aconitine co-incubation. The results showed under control conditions, [Ca2+] oscillations were irregular but relatively stable, occasionally accompanied by small calcium sparks. After incubation of the cultures with aconitine, high frequency [Ca2+] oscillations emerged in both nuclear and cytoplasmic regions, whereas typical calcium sparks disappeared and the average [Ca2+] in the cytoplasm of the cardiomyocyte did not change significantly. In AAP-treated cultures, intracellular [Ca2+] oscillation also changed, with periodic frequency, increased amplitudes and prolonged duration of calcium sparks. These patterns were not altered significantly by subsequent aconitine incubation. The basal value of [Ca2+] in nuclear region was higher than that in the cytoplasmic region. In the presence or absence of drugs, the [Ca2+] oscillated synchronously in both the nuclear and cytoplasmic regions of the same cardiomyocyte. It was concluded that although oscillating strenuously at high frequency, the average [Ca2+] in the cytoplasm of cardiomyocyte did not change significantly after aconitine incubation, compared to the controls. The observations indicate that aconitine induces the changes in [Ca2+] oscillation frequency other than the Ca2+ overload.

Endothelin-1 enhances nuclear Ca2+ transients in atrial myocytes through Ins(1,4,5)P3-dependent Ca2+ release from perinuclear Ca2+ stores

Nuclear Ca2+ plays a key role in the regulation of gene expression. Inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3)] might be an important regulator of nuclear Ca2+ but its contribution to nuclear Ca2+ signalling in adult cardiomyocytes remains elusive. We tested the hypothesis that endothelin-1 enhances nuclear Ca2+ concentration transients (CaTs) in rabbit atrial myocytes through Ins(1,4,5)P3-induced Ca(2+) release from perinuclear stores. Cytoplasmic and nuclear CaTs were measured simultaneously in electrically stimulated atrial myocytes using confocal Ca2+ imaging. Nuclear CaTs were significantly slower than cytoplasmic CaTs, indicative of compartmentalisation of intracellular Ca2+ signalling. Endothelin-1 elicited a preferential (10 nM) or a selective (0.1 nM) increase in nuclear versus cytoplasmic CaTs. This effect was abolished by inhibition of endothelin-1 receptors, phospholipase C and Ins(1,4,5)P3 receptors. Fractional Ca2+ release from the sarcoplasmic reticulum and perinuclear stores was increased by endothelin-1 at an otherwise unaltered Ca2+ load. Comparable increases of cytoplasmic CaTs induced by beta-adrenoceptor stimulation or elevation of extracellular Ca2+ could not mimic the endothelin-1 effects on nuclear CaTs, suggesting that endothelin-1 specifically modulates nuclear Ca2+ signalling. Thus, endothelin-1 enhances nuclear CaTs in atrial myocytes by increasing fractional Ca2+ release from perinuclear stores. This effect is mediated by the coupling of endothelin receptor A to PLC-Ins(1,4,5)P3 signalling and might contribute to excitation-transcription coupling.

Aconitine alters connexin43 phosphorylation status and [Ca2+] oscillation patterns in cultured ventricular myocytes of neonatal rats

Aconitine, a highly poisonous type of alkaloid, has a widespread effect in stimulating the membranes of cardiomyocyte. However, other effects of aconitine on cardiomyocyte are unknown. In this study, we investigated whether aconitine also affects the phosphorylation status of connexin43 (Cx43) and intracellular [Ca(2+)] oscillation patterns in cultured ventricular myocytes of neonatal rats. As determined by Western blot analysis, a decreased percentage (47.68+/-2.29%) of phosphorylated Cx43 (P-Cx43) and a concomitant increased percentage (52.32+/-2.29%) of nonphosphorylated Cx43 (NP-Cx43) were found in aconitine-treated cultures, compared to the controls (82.77+/-2.04% for P-Cx43 and 17.23+/-2.04% for NP-Cx43). Quantitative immunofluorescent microscopy revealed similar changes in phosphorylation status occurring in Cx43 containing gap junctions in the cultures under the same treatment conditions. Real-time laser scanning microscopy indicated that intracellular [Ca(2+)] oscillations were relatively stable in control cultures, with occasional calcium sparks; after being treated with aconitine, high frequency [Ca(2+)] oscillations emerged, whereas typical calcium sparks disappeared. Furthermore, Western blot analysis revealed that, after aconitine treatment, the amount of phosphorylated PKCalpha decreased significantly. These observations suggest that aconitine not only induces dephosphorylation of Cx43 and PKCalpha, but also alters intracellular [Ca(2+)] oscillation patterns in cultured cardiomyocytes.

Whole-field fluorescence microscope with digital micromirror device: imaging of biological samples

We have developed a whole-field fluorescence microscope equipped with a Digital Micromirror Device to acquire optically sectioned images by using the fringe-projection technique and the phase-shift method. This system allows free control of optical sectioning strength through computer-controlled alteration of the fringe period projected onto a sample. We have employed this system to image viable cells expressing fluorescent proteins and discussed its biological applications.

Optical Bioimaging: From Living Tissue to a Single Molecule: Atrio-Ventricular Difference in Myocardial Excitation-Contraction Coupling — Sequential Versus Simultaneous Activation of SR Ca2+ Release Units —

Rapid-scanning cofocal microscopy has been applied to the analysis of early phase Ca(2+) transients in ventricular and atrial cardiomyocytes. On electrical stimulation of ventricular myocytes, Ca(2+) concentration begins to rise earliest at the Z-line level and becomes uniform throughout the cytoplasm within about 10 ms after the onset of the action potential; transsarcolemmal Ca(2+) influx triggers Ca(2+) release from release sites on the junctional sarcoplasmic reticulum (SR) coupled to T-tubules at the Z-line throughout the cytoplasm. In atrial myocytes lacking the T-tubular network, transsarcolemmal Ca(2+) influx during an action potential triggers SR Ca(2+) release only at subsarcolemmal region. SR Ca(2+) release then spreads towards the central region of the cell through a propagated Ca(2+)-induced-Ca(2+) release mechanism. The atrio-ventricular difference in excitation-contraction coupling mechanisms underlies some of the atrio-ventricular difference in response to physiological and pharmacological stimuli.

[Isolated atrial tissue preparation for evaluation of cardioactive agents]

Isolated atrial tissue preparations provide convenient models for studying drug effects on the myocardium. However, there are several points we must be aware of. Interventions which change the beating rate also affect contractile force (Starling's Law). The membrane currents involved in the action potential as well as the excitation-contraction mechanisms differ between the atria and ventricle. Some membrane currents present only in the sino-atrial node and atrial myocardium may provide targets for novel bradycardiac agents and anti-atrial fibrillatory agents, respectively. The atrial tissue contains non-myocardial cells such as autonomic neurons and endocardial endothelial cells, which may be involved in the responses to various pharmacological stimuli.

Pharmacological properties of excitation-contraction mechanisms in isolated mouse left atria

Effects of 4-aminopyridine (4-AP), nicardipine and ryanodine on the action potential and contractile force were examined in isolated mouse left atria. The mouse left atria had an action potential with an extremely short duration and two phases of repolarization; action potential duration at 50% repolarization was 6.7 +/- 0.4 ms (n = 15). The action potential duration, as well as contractile force, was increased by 4-AP (at 100 micromol/l and 1 mmol/l). Nicardipine (3 micromol/l), which is known to greatly reduce the contractile force in atria of most other experimental animal species, had no significant effect on the action potential and decreased contractile force by only 40% in mouse atria. Ryanodine (10 nmol/l) decreased the contractile force by 90% of basal value. At 100 nmol/l, ryanodine slightly affected the action potential configuration, which could be explained by indirect effects through inhibition of Ca(2+) release from the sarcoplasmic reticulum. The extremely short action potential duration and the highly sarcoplasmic reticulum-dependent contraction of the mouse atria appear to underlie its unique response to agonists.

Role of sarcoplasmic reticulum in myocardial contraction of neonatal and adult mice

Changes in action potential parameters by and inotropic responses to nicardipine, verapamil, ryanodine and cyclopiazonic acid were examined in isolated ventricular myocardial preparations from neonatal and adult mice. The action potential of both neonatal and adult mice had a unique configuration with little evidence of a plateau at depolarized membrane potential; the action potential duration was significantly larger in neonatal preparations. Nicardipine had no effect on action potential parameters in the adult while it significantly shortened the action potential duration at 50% repolarization in the neonate. Ryanodine significantly shortened the action potential duration at 80% repolarization at both ages: the shortening was significantly larger in the adult when compared with the neonate. The contraction of ventricular preparations from adult mice were relatively resistant to nicardipine and verapamil. Nicardipine or verapamil, even at 10(-5) M, only decreased the contractile force to 70% of control values; the decrease was much less than that reported in other experimental species such as chick, guinea pig or rabbit. In the neonate, 10(-5) M nicardipine or verapamil decreased the contractile force to 30% of control values. Ryanodine had a potent negative inotropic effect both in the neonate and adult; the effect was significantly larger in the adult. Cyclopiazonic acid produced a decrease in contractile force and prolongation of the time required for relaxation; both effects were significantly larger in the adult. These results suggest that the contraction of the adult mouse myocardium is highly dependent on SR function and less dependent on transsarcolemmal Ca2+ influx when compared with the myocardium of the neonatal mouse and that of other species.

Effects of ryanodine and cyclopiazonic acid on skinned fibers of ventricular myocardium from neonatal and adult rats

1. We examined the effects of ryanodine and cyclopiazonic acid (CPA) on Ca2+ release from myocardial sarcoplasmic reticulum (SR) with skinned fibers of neonatal rat ventricular myocardium. 2. Both ryanodine and CPA concentration dependently reduced the caffeine-induced tension in skinned fibers with functional SR preserved; 1 microM ryanodine and 20 microM CPA reduced the caffeine-induced tension to less than 20% of control values. 3. Both agents had no effect on the Ca(2+)-tension relation of skinned fibers without functional SR. 4. These results suggest that ryanodine and CPA inhibit Ca2+ release from the SR and Ca2+ uptake into it in neonatal myocardia. 5. Thus, less-negative inotropic effects of ryanodine and CPA on neonatal myocardia compared with those on adult myocardia (Agata et al., 1993; Tanaka and Shigenobu, 1989) could not be ascribed to lack of drug effects on the SR per se.

Intrasarcomere [Ca2+] gradients and their spatio-temporal relation to Ca2+ sparks in rat cardiomyocytes

1. Line-scan analyses of spontaneous Ca2+ sparks, non-propagating local rises in Ca2+ concentration, and the early phase of Ca2+ transients in cardiomyocytes were performed with a rapid-scanning laser confocal microscope (Nikon RCM8000) and fluo-3. 2. On electrical stimulation, points at which rise in Ca2+ began earliest were observed at regular spacings of 1.82 +/- 0.26 micron (mean +/- S.D.) along the longitudinal axis of the cell. The points were heavily stained with di-2-ANEPEQ, which stains the T-tubules, indicating that they were at the Z-line. 3. The points where spontaneous Ca2+ sparks originated coincided with the points which showed faster Ca2+ elevation, i.e. the Z-line. 4. In some cases where a Ca2+ spark had occurred within about 30 ms before the evoked Ca2+ transient, fast elevation of Ca2+ was not observed at the corresponding Z-line, indicating the presence of a refractory period in Ca2+ release from the SR. 5. The present results provide visual evidence for Ca2+ release from the junctional sarcoplasmic reticulum in cardiomyocytes. The presence of a refractory period in Ca2+ release after Ca2+ sparks provided new evidence that the normal Ca2+ transient may be the summation of Ca2+ sparks.

Two-dimensional millisecond analysis of intracellular Ca2+ sparks in cardiac myocytes by rapid scanning confocal microscopy: increase in amplitude by isoproterenol

Two dimensional images of myocardial Ca2+ sparks, non-propagating local rises in cytoplasmic Ca2+ concentration, were obtained at 4 msec intervals with a rapid-scanning confocal laser microscope, Nikon RCM 8000, and fluo-3. Spontaneous Ca2+ sparks were observed at apparently random sites throughout the cytoplasm of rat ventricular cells. The duration of sparks was 30 to 40 msec and the time to peak intensity about 10 msec. Ryanodine (1 microM) completely inhibited Ca2+ sparks while nicardipine (3 microM) had no effect. Isoproterenol (1 microM) had no effect on the frequency and distribution of Ca2+ sparks but significantly increased their amplitude. These results suggest that myocardial Ca2+ sparks are the result of spontaneous release of Ca2+ from the sarcoplasmic reticulum and that beta-adrenergic stimulation may result in functional modification of the ryanodine receptor channel.