Developmental aspects of cardiac Ca(2+) signaling: interplay between RyR- and IP(3)R-gated Ca(2+) stores

TRPC channels blockade abolishes endotoxemic cardiac dysfunction by hampering intracellular inflammation and Ca2+ leakage

Intracellular Ca²⁺ dysregulation is a key marker in septic cardiac dysfunction; however, regulation of the classic Ca²⁺ regulatory modules cannot successfully abolish this symptom. Here we show that the knockout of transient receptor potential canonical (TRPC) channel isoforms TRPC1 and TRPC6 can ameliorate LPS-challenged heart failure and prolong survival in mice. The LPS-triggered Ca²⁺ release from the endoplasmic reticulum both in cardiomyocytes and macrophages is significantly inhibited by Trpc1 or Trpc6 knockout. Meanwhile, TRPC's molecular partner - calmodulin - is uncoupled during Trpc1 or Trpc6 deficiency and binds to TLR4's Pococurante site and atypical isoleucine-glutamine-like motif to block the inflammation cascade. Blocking the C-terminal CaM/IP3R binding domain in TRPC with chemical inhibitor could obstruct the Ca²⁺ leak and TLR4-mediated inflammation burst, demonstrating a cardioprotective effect in endotoxemia and polymicrobial sepsis. Our findings provide insight into the pathogenesis of endotoxemic cardiac dysfunction and suggest a novel approach for its treatment.

Ca2+ signaling of human pluripotent stem cells-derived cardiomyocytes as compared to adult mammalian cardiomyocytes

Human induced pluripotent stem cells derived cardiomyocytes (hiPSC-CMs) have been extensively used for in vitro modeling of human cardiovascular disease, drug screening and pharmacotherapy, but little rigorous studies have been reported on their biophysical or Ca2+ signaling properties. There is also considerable concern as to the level of their maturity and whether they can serve as reliable models for adult human cardiac myocytes. Ultrastructural difference such as lack of t-tubular network, their polygonal shapes, disorganized sarcomeric myofilament, and their rhythmic automaticity, among others, have been cited as evidence for immaturity of hiPSC-CMs. In this review, we will deal with Ca2+ signaling, its regulation, and its stage of maturity as compared to the mammalian adult cardiomyocytes. We shall summarize the data on functional aspects of Ca2+signaling and its parameters that include: L-type calcium channel (Cav1.2), ICa-induced Ca2+release, CICR, and its parameters, cardiac Na/Ca exchanger (NCX1), the ryanodine receptors (RyR2), sarco-reticular Ca2+pump, SERCA2a/PLB, and the contribution of mitochondrial Ca2+ to hiPSC-CMs excitation-contraction (EC)-coupling as compared with adult mammalian cardiomyocytes. The comparative studies suggest that qualitatively hiPSC-CMs have similar Ca2+signaling properties as those of adult cardiomyocytes, but quantitative differences do exist. This review, we hope, will allow the readers to judge for themselves to what extent Ca2+signaling of hiPSC-CMs represents the adult form of this signaling pathway, and whether these cells can be used as good models of human cardiomyocytes.

The GSTM2 C-Terminal Domain Depresses Contractility and Ca2+ Transients in Neonatal Rat Ventricular Cardiomyocytes

The cardiac ryanodine receptor (RyR2) is an intracellular ion channel that regulates Ca²⁺ release from the sarcoplasmic reticulum (SR) during excitation–contraction coupling in the heart. The glutathione transferases (GSTs) are a family of phase II detoxification enzymes with additional functions including the selective inhibition of RyR2, with therapeutic implications. The C-terminal half of GSTM2 (GSTM2C) is essential for RyR2 inhibition, and mutations F157A and Y160A within GSTM2C prevent the inhibitory action. Our objective in this investigation was to determine whether GSTM2C can enter cultured rat neonatal ventricular cardiomyocytes and influence contractility. We show that oregon green-tagged GSTM2C (at 1 μM) is internalized into the myocytes and it reduces spontaneous contraction frequency and myocyte shortening. Field stimulation of myocytes evoked contraction in the same percentage of myocytes treated either with media alone or media plus 15 μM GSTM2C. Myocyte shortening during contraction was significantly reduced by exposure to 15 μM GSTM2C, but not 5 and 10 μM GSTM2C and was unaffected by exposure to 15 μM of the mutants Y160A or F157A. The amplitude of the Ca²⁺ transient in the 15 μM GSTM2C - treated myocytes was significantly decreased, the rise time was significantly longer and the decay time was significantly shorter than in control myocytes. The Ca²⁺ transient was not altered by exposure to Y160A or F157A. The results are consistent with GSTM2C entering the myocytes and inhibiting RyR2, in a manner that indicates a possible therapeutic potential for treatment of arrhythmia in the neonatal heart.

Calcium signaling in human stem cell-derived cardiomyocytes: Evidence from normal subjects and CPVT afflicted patients

Derivation of cardiomyocyte cell lines from human fibroblasts (induced pluripotent stem cells, iPSCs) has made it possible not only to investigate the electrophysiological and Ca(2+) signaling properties of these cells, but also to determine the altered electrophysiological and Ca(2+)-signaling profiles of such cells lines derived from patients expressing mutation-inducing pathologies. This approach has the potential of generating in vitro human models of cardiovascular diseases where cellular pathology can be investigated in detail and possibly specific pharmacotherapy developed. Although this approach has been applied to a number of mutations in channel proteins that cause arrhythmias, there are only few detailed reports addressing Ca(2+) signaling pathologies beyond measurements of Ca(2+) transients in intact non-voltage clamped cells. Unfortunately, full understanding of Ca(2+) signaling pathologies remains elusive, not only because of the plethora of Ca(2+) signaling proteins defects that cause arrhythmias and cardiomyopathies, but also because detailed functional properties of Ca(2+) signaling proteins are difficult to obtain. Catecholaminergic polymorphic ventricular tachycardia (CPVT1) is a malignant inherited arrhythmogenic disorder predominantly caused by mutations in the cardiac ryanodine receptor (RyR2). Thus far over 150 mutations in RyR2 have been identified that appear to cause this arrhythmia, a number of which have been expressed and studied in transgenic mice or cell-line models. The development of human iPSC-technology makes it possible to create human heart cell-lines carrying these mutations, making detailed identification of Ca(2+) signaling defects and its specific pharmacotherapy possible. In this review we shall first briefly summarize the essential characteristics of the mammalian cardiac Ca(2+) signaling, then compare them to Ca(2+) signaling phenotypes of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) and to those of rat neonatal cardiomyocytes, and categorize the possible variance in Ca(2+) signaling defects caused by different CPVT-inducing mutations as expressed in hiPSC-CMs.

RNA-Mediated Reprogramming of Primary Adult Human Dermal Fibroblasts into c-kit(+) Cardiac Progenitor Cells

Cardiovascular disease is the leading cause of death in the United States. Heart failure is a common, costly, and potentially fatal condition that is inadequately managed by pharmaceuticals. Cardiac repair therapies are promising alternative options. A potential cardiac repair therapy involves reprogramming human fibroblasts toward an induced cardiac progenitor-like state. We developed a clinically useful and safer reprogramming method by nonintegrative delivery of a cocktail of cardiac transcription factor-encoding mRNAs into autologous human dermal fibroblasts obtained from skin biopsies. Using this method, adult and neonatal dermal fibroblasts were reprogrammed into cardiac progenitor cells (CPCs) that expressed c-kit, Isl-1, and Nkx2.5. Furthermore, these reprogrammed CPCs differentiated into cardiomyocytes (CMs) in vitro as judged by increased expression of cardiac troponin T, α-sarcomeric actinin, RyR2, and SERCA2 and displayed enhanced caffeine-sensitive calcium release. The ability to reprogram patient-derived dermal fibroblasts into c-kit(+) CPCs and differentiate them into functional CMs provides clinicians with a potential new source of CPCs for cardiac repair from a renewable source and an alternative therapy in the treatment of heart failure.

Calcium signalling in developing cardiomyocytes: implications for model systems and disease

Adult cardiomyocytes exhibit complex Ca(2+) homeostasis, enabling tight control of contraction and relaxation. This intricate regulatory system develops gradually, with progressive maturation of specialized structures and increasing capacity of Ca(2+) sources and sinks. In this review, we outline current understanding of these developmental processes, and draw parallels to pathophysiological conditions where cardiomyocytes exhibit a striking regression to an immature state of Ca(2+) homeostasis. We further highlight the importance of understanding developmental physiology when employing immature cardiomyocyte models such as cultured neonatal cells and stem cells.

Regionally diverse mitochondrial calcium signaling regulates spontaneous pacing in developing cardiomyocytes

The quintessential property of developing cardiomyocytes is their ability to beat spontaneously. The mechanisms underlying spontaneous beating in developing cardiomyocytes are thought to resemble those of adult heart, but have not been directly tested. Contributions of sarcoplasmic and mitochondrial Ca(2+)-signaling vs. If-channel in initiating spontaneous beating were tested in human induced Pluripotent Stem cell-derived cardiomyocytes (hiPS-CM) and rat Neonatal cardiomyocytes (rN-CM). Whole-cell and perforated-patch voltage-clamping and 2-D confocal imaging showed: (1) both cell types beat spontaneously (60-140/min, at 24°C); (2) holding potentials between -70 and 0mV had no significant effects on spontaneous pacing, but suppressed action potential formation; (3) spontaneous pacing at -50mV activated cytosolic Ca(2+)-transients, accompanied by in-phase inward current oscillations that were suppressed by Na(+)-Ca(2+)-exchanger (NCX)- and ryanodine receptor (RyR2)-blockers, but not by Ca(2+)- and If-channels blockers; (4) spreading fluorescence images of cytosolic Ca(2+)-transients emanated repeatedly from preferred central cellular locations during spontaneous beating; (5) mitochondrial un-coupler, FCCP at non-depolarizing concentrations (∼50nM), reversibly suppressed spontaneous pacing; (6) genetically encoded mitochondrial Ca(2+)-biosensor (mitycam-E31Q) detected regionally diverse, and FCCP-sensitive mitochondrial Ca(2+)-uptake and release signals activating during INCX oscillations; (7) If-channel was absent in rN-CM, but activated only negative to -80mV in hiPS-CM; nevertheless blockers of If-channel failed to alter spontaneous pacing.

An integrated mechanism of cardiomyocyte nuclear Ca(2+) signaling

In cardiomyocytes, Ca(2+) plays a central role in governing both contraction and signaling events that regulate gene expression. Current evidence indicates that discrimination between these two critical functions is achieved by segregating Ca(2+) within subcellular microdomains: transcription is regulated by Ca(2+) release within nuclear microdomains, and excitation-contraction coupling is regulated by cytosolic Ca(2+). Accordingly, a variety of agonists that control cardiomyocyte gene expression, such as endothelin-1, angiotensin-II or insulin-like growth factor-1, share the feature of triggering nuclear Ca(2+) signals. However, signaling pathways coupling surface receptor activation to nuclear Ca(2+) release, and the phenotypic responses to such signals, differ between agonists. According to earlier hypotheses, the selective control of nuclear Ca(2+) signals by activation of plasma membrane receptors relies on the strategic localization of inositol trisphosphate receptors at the nuclear envelope. There, they mediate Ca(2+) release from perinuclear Ca(2+) stores upon binding of inositol trisphosphate generated in the cytosol, which diffuses into the nucleus. More recently, identification of such receptors at nuclear membranes or perinuclear sarcolemmal invaginations has uncovered novel mechanisms whereby agonists control nuclear Ca(2+) release. In this review, we discuss mechanisms for the selective control of nuclear Ca(2+) signals with special focus on emerging models of agonist receptor activation.

PLCδ1 protein rescues ischemia-reperfused heart by the regulation of calcium homeostasis

Myocardial Ca(2+) overload induced by ischemia/reperfusion (I/R) is a major element of myocardial dysfunction in heart failure. Phospholipase C (PLC) plays important roles in the regulation of the phosphoinositol pathway and Ca(2+) homeostasis in various types of cells. Here, we investigated the protective role of PLCδ1 against myocardial I/R injury through the regulation of Ca(2+) homeostasis. To investigate its role, PLCδ1 was fused to Hph1, a cell-permeable protein transduction domain (PTD), and treated into rat neonatal cardiomyocytes and rat hearts under respective hypoxia-reoxygenation (H/R) and ischemia-reperfusion conditions. Treatment with Hph1-PLCδ1 significantly inhibited intracellular Ca(2+) overload, reactive oxygen species generation, mitochondrial permeability transition pore opening, and mitochondrial membrane potential elevation in H/R neonatal cardiomyocytes, resulting in the inhibition of apoptosis. Intravenous injections of Hph1-PLCδ1 in rats with I/R-injured myocardium caused significant reductions in infarct size and apoptosis and also improved systolic and diastolic cardiac functioning. Furthermore, a small ions profile obtained using time-of-flight secondary ion mass spectrometry showed that treatment with Hph1-PLCδ1 leads to significant recovery of calcium-related ions toward normal levels in I/R-injured myocardium. These results suggest that Hph1-PLCδ1 may manifest as a promising cardioprotective drug due to its inhibition of the mitochondrial apoptotic pathway in cells suffering from I/R injury.

Inositol 1, 4, 5-trisphosphate receptors and human left ventricular myocytes

BACKGROUND

Little is known about the function of inositol 1,4,5-trisphosphate receptors (IP3Rs) in the adult heart experimentally. Moreover, whether these Ca(2+) release channels are present and play a critical role in human cardiomyocytes remains to be defined. IP3Rs may be activated after Gαq-protein-coupled receptor stimulation, affecting Ca(2+) cycling, enhancing myocyte performance, and potentially favoring an increase in the incidence of arrhythmias.

METHODS AND RESULTS

IP3R function was determined in human left ventricular myocytes, and this analysis was integrated with assays in mouse myocytes to identify the mechanisms by which IP3Rs influence the electric and mechanical properties of the myocardium. We report that IP3Rs are expressed and operative in human left ventricular myocytes. After Gαq-protein-coupled receptor activation, Ca(2+) mobilized from the sarcoplasmic reticulum via IP3Rs contributes to the decrease in resting membrane potential, prolongation of the action potential, and occurrence of early afterdepolarizations. Ca(2+) transient amplitude and cell shortening are enhanced, and extrasystolic and dysregulated Ca(2+) elevations and contractions become apparent. These alterations in the electromechanical behavior of human cardiomyocytes are coupled with increased isometric twitch of the myocardium and arrhythmic events, suggesting that Gαq-protein-coupled receptor activation provides inotropic reserve, which is hampered by electric instability and contractile abnormalities. Additionally, our findings support the notion that increases in Ca(2+) load by IP3Rs promote Ca(2+) extrusion by forward-mode Na(+)/Ca(2+) exchange, an important mechanism of arrhythmic events.

CONCLUSIONS

The Gαq-protein/coupled receptor/IP3R axis modulates the electromechanical properties of the human myocardium and its propensity to develop arrhythmias.

Differential activation of cultured neonatal cardiomyocytes by plasmalemmal versus intracellular G protein-coupled receptor 55

The L-α-lysophosphatidylinositol (LPI)-sensitive receptor GPR55 is coupled to Ca(2+) signaling. Low levels of GPR55 expression in the heart have been reported. Similar to other G protein-coupled receptors involved in cardiac function, GPR55 may be expressed both at the sarcolemma and intracellularly. Thus, to explore the role of GPR55 in cardiomyocytes, we used calcium and voltage imaging and extracellular administration or intracellular microinjection of GPR55 ligands. We provide the first evidence that, in cultured neonatal ventricular myocytes, LPI triggers distinct signaling pathways via GPR55, depending on receptor localization. GPR55 activation at the sarcolemma elicits, on one hand, Ca(2+) entry via L-type Ca(2+) channels and, on the other, inositol 1,4,5-trisphosphate-dependent Ca(2+) release. The latter signal is further amplified by Ca(2+)-induced Ca(2+) release via ryanodine receptors. Conversely, activation of GPR55 at the membrane of intracellular organelles promotes Ca(2+) release from acidic-like Ca(2+) stores via the endolysosomal NAADP-sensitive two-pore channels. This response is similarly enhanced by Ca(2+)-induced Ca(2+) release via ryanodine receptors. Extracellularly applied LPI produces Ca(2+)-independent membrane depolarization, whereas the Ca(2+) signal induced by intracellular microinjection of LPI converges to hyperpolarization of the sarcolemma. Collectively, our findings point to GPR55 as a novel G protein-coupled receptor regulating cardiac function at two cellular sites. This work may serve as a platform for future studies exploring the potential of GPR55 as a therapeutic target in cardiac disorders.

Ca²⁺ waves in the heart

Ca(2+) waves were probably first observed in the early 1940s. Since then Ca(2+) waves have captured the attention of an eclectic mixture of mathematicians, neuroscientists, muscle physiologists, developmental biologists, and clinical cardiologists. This review discusses the current state of mathematical models of Ca(2+) waves, the normal physiological functions Ca(2+) waves might serve in cardiac cells, as well as how the spatial arrangement of Ca(2+) release channels shape Ca(2+) waves, and we introduce the idea of Ca(2+) phase waves that might provide a useful framework for understanding triggered arrhythmias.

The effect of microgrooved culture substrates on calcium cycling of cardiac myocytes derived from human induced pluripotent stem cells

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) have been widely proposed as in vitro models of myocardial physiology and disease. A significant obstacle, however, is their immature phenotype. We hypothesised that Ca(2+) cycling of iPSC-CM is influenced by culture conditions and can be manipulated to obtain a more mature cellular behaviour. To test this hypothesis we seeded iPSC-CM onto fibronectin coated microgrooved polydimethylsiloxane (PDMS) scaffolds fabricated using photolithography, or onto unstructured PDMS membrane. After two weeks in culture, the structure and function of iPSC-CM were studied. PDMS microgrooved culture substrates brought about cellular alignment (p < 0.0001) and more organised sarcomere. The Ca(2+) cycling properties of iPSC-CM cultured on these substrates were significantly altered with a shorter time to peak amplitude (p = 0.0002 at 1 Hz), and more organised sarcoplasmic reticulum (SR) Ca(2+) release in response to caffeine (p < 0.0001), suggesting improved SR Ca(2+) cycling. These changes were not associated with modifications in gene expression. Whilst structured tissue culture may make iPSC-CM more representative of adult myocardium, further construct development and characterisation is required to optimise iPSC-CM as a model of adult myocardium.

Stroma cell-derived factor-1α signaling enhances calcium transients and beating frequency in rat neonatal cardiomyocytes

Stroma cell-derived factor-1α (SDF-1α) is a cardioprotective chemokine, acting through its G-protein coupled receptor CXCR4. In experimental acute myocardial infarction, administration of SDF-1α induces an early improvement of systolic function which is difficult to explain solely by an anti-apoptotic and angiogenic effect. We wondered whether SDF-1α signaling might have direct effects on calcium transients and beating frequency.

Primary rat neonatal cardiomyocytes were culture-expanded and characterized by immunofluorescence staining. Calcium sparks were studied by fluorescence microscopy after calcium loading with the Fluo-4 acetoxymethyl ester sensor. The cardiomyocyte enriched cellular suspension expressed troponin I and CXCR4 but was vimentin negative. Addition of SDF-1α in the medium increased cytoplasmic calcium release. The calcium response was completely abolished by using a neutralizing anti-CXCR4 antibody and partially suppressed and delayed by preincubation with an inositol triphosphate receptor (IP₃R) blocker, but not with a ryanodine receptor (RyR) antagonist. Calcium fluxes induced by caffeine, a RyR agonist, were decreased by an IP₃R blocker. Treatment with forskolin or SDF-1α increased cardiomyocyte beating frequency and their effects were additive. In vivo, treatment with SDF-1α increased left ventricular dP/dtmax.

These results suggest that in rat neonatal cardiomyocytes, the SDF-1α/CXCR4 signaling increases calcium transients in an IP₃-gated fashion leading to a positive chronotropic and inotropic effect.

Cardiac progenitor cells engineered with Pim-1 (CPCeP) develop cardiac phenotypic electrophysiological properties as they are co-cultured with neonatal myocytes

Stem cell transplantation has been successfully used for amelioration of cardiomyopathic injury using adult cardiac progenitor cells (CPC). Engineering of mouse CPC with the human serine/threonine kinase Pim-1 (CPCeP) enhances regeneration and cell survival in vivo, but it is unknown if such apparent lineage commitment is associated with maturation of electrophysiological properties and excitation-contraction coupling. This study aims to determine electrophysiology and Ca(2+)-handling properties of CPCeP using neonatal rat cardiomyocyte (NRCM) co-culture to promote cardiomyocyte lineage commitment. Measurements of membrane capacitance, dye transfer, expression of connexin 43 (Cx43), and transmission of ionic currents (I(Ca), I(Na)) from one cell to the next suggest that a subset of co-cultured CPCeP and NRCM becomes connected via gap junctions. Unlike NRCM, CPCeP had no significant I(Na), but expressed nifedipine-sensitive I(Ca) that could be measured more consistently with Ba(2+) as permeant ion using ramp-clamp protocols than with Ca(2+) and step-depolarization protocols. The magnitude of I(Ca) in CPCeP increased during culture (4-7 days vs. 1-3 days) and was larger in co-cultures with NRCM and with NRCM-conditioned medium, than in mono-cultured CPCeP. I(Ca) was virtually absent in CPC without engineered expression of Pim-1. Caffeine and KCl-activated Ca(2+)-transients were significantly present in co-cultured CPCeP, but smaller than in NRCM. Conversely, ATP-induced (IP(3)-mediated) Ca(2+) transients were larger in CPCeP than in NRCM. I(NCX) and I(ATP) were expressed in equivalent densities in CPCeP and NRCM. These in vitro studies suggest that CPCeP in co-culture with NRCM: a) develop I(Ca) current and Ca(2+) signaling consistent with cardiac lineage, b) form electrical connections via Cx43 gap junctions, and c) respond to paracrine signals from NRCM. These properties may be essential for durable and functional myocardial regeneration under in vivo conditions.

Contribution of sarcoplasmic reticulum Ca²+ release and Ca²+ transporters on sarcolemmal channels to Ca²+ transient in fetal mouse heart

Sarcoplasmic reticulum (SR) Ca release has been shown not to be the predominant mechanism responsible for excitation-contraction (E-C) coupling in fetal myocytes. However, most of the studies have been conducted either on primary cultures or acutely isolated cells, in which an apparent reduction of ryanodine receptor density have been reported. We aimed to elucidate the contribution of SR Ca release and Ca transporters on sarcolemmal channels to Ca transients in fetal mouse whole hearts. On embryonic day 13.5, ryanodine significantly reduced the amplitude of the Ca transient to 27.2 ± 4.4% of the control, and both nickel and SEA0400 significantly prolonged the time to peak from 84 ± 2 ms to 140 ± 5 ms and 129 ± 6 ms, respectively, whereas nifedipine did not alter it. Therefore, at early fetal stages, SR Ca release should be an important component of E-C coupling, and T-type Ca channel and reverse mode sodium-calcium exchanger (NCX)-mediated SR Ca release could be the predominant contributors. Using embryonic mouse cultured cardiomyocytes, we showed that both nifedipine and nickel inhibited the ability of NCX to extrude Ca from the cytosol. From these results, we propose a novel idea concerning E-C coupling in immature heart.