Palmitoylation of the pore-forming subunit of Ca(v)1.2 controls channel voltage sensitivity and calcium transients in cardiac myocytes

Mammalian voltage-activated L-type Ca2+ channels, such as Ca(v)1.2, control transmembrane Ca2+ fluxes in numerous excitable tissues. Here, we report that the pore-forming α1C subunit of Ca(v)1.2 is reversibly palmitoylated in rat, rabbit, and human ventricular myocytes. We map the palmitoylation sites to two regions of the channel: The N terminus and the linker between domains I and II. Whole-cell voltage clamping revealed a rightward shift of the Ca(v)1.2 current-voltage relationship when α1C was not palmitoylated. To examine function, we expressed dihydropyridine-resistant α1C in human induced pluripotent stem cell-derived cardiomyocytes and measured Ca2+ transients in the presence of nifedipine to block the endogenous channels. The transients generated by unpalmitoylatable channels displayed a similar activation time course but significantly reduced amplitude compared to those generated by wild-type channels. We thus conclude that palmitoylation controls the voltage sensitivity of Ca(v)1.2. Given that the identified Ca(v)1.2 palmitoylation sites are also conserved in most Ca(v)1 isoforms, we propose that palmitoylation of the pore-forming α1C subunit provides a means to regulate the voltage sensitivity of voltage-activated Ca2+ channels in excitable cells.

SUMOylation does not affect cardiac troponin I stability but alters indirectly the development of force in response to Ca2

Post-translational modification of the myofilament protein troponin I by phosphorylation is known to trigger functional changes that support enhanced contraction and relaxation of the heart. We report for the first time that human troponin I can also be modified by SUMOylation at lysine 177. Functionally, TnI SUMOylation is not a factor in the development of passive and maximal force generation in response to calcium, however this modification seems to act indirectly by preventing SUMOylation of other myofilament proteins to alter calcium sensitivity and cooperativity of myofilaments. Utilising a novel, custom SUMO site-specific antibody that recognises only the SUMOylated form of troponin I, we verify that this modification occurs in human heart and that it is upregulated during disease.

Phosphodiesterase type 4 anchoring regulates cAMP signaling to Popeye domain-containing proteins

Cyclic AMP is a ubiquitous second messenger used to transduce intracellular signals from a variety of Gs-coupled receptors. Compartmentalisation of protein intermediates within the cAMP signaling pathway underpins receptor-specific responses. The cAMP effector proteins protein-kinase A and EPAC are found in complexes that also contain phosphodiesterases whose presence ensures a coordinated cellular response to receptor activation events. Popeye domain containing (POPDC) proteins are the most recent class of cAMP effectors to be identified and have crucial roles in cardiac pacemaking and conduction. We report the first observation that POPDC proteins exist in complexes with members of the PDE4 family in cardiac myocytes. We show that POPDC1 preferentially binds the PDE4A sub-family via a specificity motif in the PDE4 UCR1 region and that PDE4s bind to the Popeye domain of POPDC1 in a region known to be susceptible to a mutation that causes human disease. Using a cell-permeable disruptor peptide that displaces the POPDC1-PDE4 complex we show that PDE4 activity localized to POPDC1 modulates cycle length of spontaneous Ca²⁺ transients firing in intact mouse sinoatrial nodes.

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POPDC1 forms a complex with type 4 phosphodiesterases (PDE4s) in cardiac myocytes.

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POPDC1 binds PDE4 enzymes in the Upstream Conserved Region 1 (UCR1) domain.

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The PDE4 binding motif within the Popeye domain lies in a region that harbours a mutation, which underpins human disease.

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Disruption of the POPDC1-PDE4 complex modulates the cycle length of spontaneous Ca2+ transients in the sinoatrial node.

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Disruption of the POPDC1-PDE4 complex causes a significant prolongation of the action potential repolarization phase.

Abstract 089: Role of Nox5 in Systemic Vascular Dysfunction in Ischemic Heart Disease

Patients with coronary microvascular dysfunction (CMD), a potential cause of heart ischemia, have systemic vascular dysfunction, characterized by increased vascular contraction to ET-1 and a thromboxane A2 analogue (U46619). Nox5 regulates vascular contraction and is involved in cardiovascular diseases. In our study, we questioned whether Nox5 plays a role in systemic vascular dysfunction in heart ischemia. As Nox5 expression has been described in the cardiovascular system of rabbits, a model of ischaemic cardiomyopathy (IC) was used. Coronary artery ligation was performed in Male New Zealand White rabbits. After 8 weeks, skin and mesenteric arteries were isolated and vascular function assessed by wire myography. Vascular contraction to NA (EMax %KCl: 122±4 vs sham 97±3.7) and U46619 (EMax %KCl: 82±3 vs sham 67±4) were exacerbated in skin arteries from IC (p<0.05); an effect blocked by tiron (antioxidant, 10 μM) and melittin (nox5 inhibitor, 0.1 μM). In mesenteric arteries from IC animals, NA (EMax %KCl: 108±3 vs sham 98±7) and ET-1 (EMax %KCl: 103±3 vs sham 81±4) induced contraction were increased in a Nox5-ROS-dependent manner (p<0.05). No differences were observed in mRNA levels of Cav1.2 and IP3R Ca 2+ channels, but an increase in RyR was observed (2^-ddCT: 1.67±0.15 vs sham 0.98±0.08) in VSMCs isolated from IC animals. Peroxiredoxin (Prdx), antioxidant, mRNA was increased in IC (2^-ddCT: 1.95±0.4 vs sham 0.88±0.1, p<0.05). Conoidin A, an inhibitor of Prdx oxidation, reduced vascular contraction to NA in arteries from IC animals (EMax %KCl: 95±4, p<0.05). In subjects with CMD, we assessed total number of microparticles (MP) as biomarkers of vascular dysfunction. MPs were increased in CMD subjects (x10 11 /mL: 4.8±0.6 vs control 1.75±0.2), where Nox5 expression was also increased (AU: 0.11±0.02 vs control MP 0.03±0.006) (p<0.05). In separate studies, we exposed WT control arteries to MPs from WT and Nox5-expressing mice before assessing contraction. MPs from Nox5 mice increased contraction to a higher level of that observed with MPs from WT mice (EMax %KCl: 106±2 vs WT 96±2, p<0.05). In our study, we identify Nox5 as a regulator of systemic vascular dysfunction in ischemic heart diseases, through mechanisms that may involve ROS, Prdx oxidation and MPs.

3D dSTORM imaging reveals novel detail of ryanodine receptor localization in rat cardiac myocytes

KEY POINTS

Using 3D direct stochastic optical reconstruction microscopy (dSTORM), we developed novel approaches to quantitatively describe the nanoscale, 3D organization of ryanodine receptors (RyRs) in cardiomyocytes. Complex arrangements of RyR clusters were observed in 3D space, both at the cell surface and within the cell interior, with allocation to dyadic and non-dyadic pools. 3D imaging importantly allowed discernment of clusters overlapping in the z-axis, for which detection was obscured by conventional 2D imaging techniques. Thus, RyR clusters were found to be significantly smaller than previous 2D estimates. Ca2+ release units (CRUs), i.e. functional groupings of neighbouring RyR clusters, were similarly observed to be smaller than earlier reports. Internal CRUs contained more RyRs in more clusters than CRUs on the cell surface, and yielded longer duration Ca2+ sparks.

ABSTRACT

Cardiomyocyte contraction is dependent on Ca2+ release from ryanodine receptors (RyRs). However, the precise localization of RyRs remains unknown, due to shortcomings of imaging techniques which are diffraction limited or restricted to 2D. We aimed to determine the 3D nanoscale organization of RyRs in rat cardiomyocytes by employing direct stochastic optical reconstruction microscopy (dSTORM) with phase ramp technology. Initial observations at the cell surface showed an undulating organization of RyR clusters, resulting in their frequent overlap in the z-axis and obscured detection by 2D techniques. Non-overlapping clusters were imaged to create a calibration curve for estimating RyR number based on recorded fluorescence blinks. Employing this method at the cell surface and interior revealed smaller RyR clusters than 2D estimates, as erroneous merging of axially aligned RyRs was circumvented. Functional groupings of RyR clusters (Ca2+ release units, CRUs), contained an average of 18 and 23 RyRs at the surface and interior, respectively, although half of all CRUs contained only a single 'rogue' RyR. Internal CRUs were more tightly packed along z-lines than surface CRUs, contained larger and more numerous RyR clusters, and constituted ∼75% of the roughly 1 million RyRs present in an average cardiomyocyte. This complex internal 3D geometry was underscored by correlative imaging of RyRs and t-tubules, which enabled quantification of dyadic and non-dyadic RyR populations. Mirroring differences in CRU size and complexity, Ca2+ sparks originating from internal CRUs were of longer duration than those at the surface. These data provide novel, nanoscale insight into RyR organization and function across cardiomyocytes.

Dyadic Plasticity in Cardiomyocytes

Contraction of cardiomyocytes is dependent on sub-cellular structures called dyads, where invaginations of the surface membrane (t-tubules) form functional junctions with the sarcoplasmic reticulum (SR). Within each dyad, Ca2+ entry through t-tubular L-type Ca2+ channels (LTCCs) elicits Ca2+ release from closely apposed Ryanodine Receptors (RyRs) in the SR membrane. The efficiency of this process is dependent on the density and macroscale arrangement of dyads, but also on the nanoscale organization of LTCCs and RyRs within them. We presently review accumulating data demonstrating the remarkable plasticity of these structures. Dyads are known to form gradually during development, with progressive assembly of both t-tubules and junctional SR terminals, and precise trafficking of LTCCs and RyRs. While dyads can exhibit compensatory remodeling when required, dyadic degradation is believed to promote impaired contractility and arrythmogenesis in cardiac disease. Recent data indicate that this plasticity of dyadic structure/function is dependent on the regulatory proteins junctophilin-2, amphiphysin-2 (BIN1), and caveolin-3, which critically arrange dyadic membranes while stabilizing the position and activity of LTCCs and RyRs. Indeed, emerging evidence indicates that clustering of both channels enables "coupled gating", implying that nanoscale localization and function are intimately linked, and may allow fine-tuning of LTCC-RyR crosstalk. We anticipate that improved understanding of dyadic plasticity will provide greater insight into the processes of cardiac compensation and decompensation, and new opportunities to target the basic mechanisms underlying heart disease.

Ryanodine receptor dispersion disrupts Ca2+ release in failing cardiac myocytes

Reduced cardiac contractility during heart failure (HF) is linked to impaired Ca²⁺ release from Ryanodine Receptors (RyRs). We investigated whether this deficit can be traced to nanoscale RyR reorganization. Using super-resolution imaging, we observed dispersion of RyR clusters in cardiomyocytes from post-infarction HF rats, resulting in more numerous, smaller clusters. Functional groupings of RyR clusters which produce Ca²⁺ sparks (Ca²⁺ release units, CRUs) also became less solid. An increased fraction of small CRUs in HF was linked to augmented ‘silent’ Ca²⁺ leak, not visible as sparks. Larger multi-cluster CRUs common in HF also exhibited low fidelity spark generation. When successfully triggered, sparks in failing cells displayed slow kinetics as Ca²⁺ spread across dispersed CRUs. During the action potential, these slow sparks protracted and desynchronized the overall Ca²⁺ transient. Thus, nanoscale RyR reorganization during HF augments Ca²⁺ leak and slows Ca²⁺ release kinetics, leading to weakened contraction in this disease.

The muscle cells of the heart coordinate how they contract and relax in order to produce the heartbeat. During heart failure, these cells become less able to contract. As a result the heart becomes inefficient, pumping less blood around the body.

For the cardiac muscle cells to contract, the levels of calcium ions in the cells needs to rapidly increase. In failing hearts, these increases in calcium ion levels are smaller, slower and less well coordinated. It was not known what causes these changes, making it difficult to treat heart failure.

Calcium ions are released in cardiac muscle cells through protein channels called ryanodine receptors. These receptors form clusters that allow them to synchronize when they open and close. Could the reorganization of ryanodine receptors account for the problems seen in failing hearts? To investigate, Kolstad et al. examined rat hearts using a technique called super-resolution microscopy. This showed that the clusters of ryanodine receptors break apart during heart failure to form smaller clusters. Further experiments showed that calcium ions ‘leak’ from these smaller clusters, reducing the amount of calcium that can be released into cardiac muscle cells during each heartbeat. Released calcium also spreads between the dispersed clusters, resulting in a slower rise of the calcium levels in the cells. Both changes contribute to weakened contractions of cells in failing hearts.

Therefore, heart failure can be traced back to very small rearrangements of the ryanodine receptors. This understanding will help researchers as they investigate new ways to treat heart failure.

Spontaneous Ca2+ transients in rat pulmonary vein cardiomyocytes are increased in frequency and become more synchronous following electrical stimulation

The pulmonary veins have an external sleeve of cardiomyocytes that are a widely recognised source of ectopic electrical activity that can lead to atrial fibrillation. Although the mechanisms behind this activity are currently unknown, changes in intracellular calcium (Ca²⁺) signalling are purported to play a role. Therefore, the intracellular Ca²⁺ concentration was monitored in the pulmonary vein using fluo-4 and epifluorescence microscopy. Electrical field stimulation evoked a synchronous rise in Ca²⁺ in neighbouring cardiomyocytes; asynchronous spontaneous Ca²⁺ transients between electrical stimuli were also present. Immediately following termination of electrical field stimulation at 3 Hz or greater, the frequency of the spontaneous Ca²⁺ transients was increased from 0.45 ± 0.06 Hz under basal conditions to between 0.59 ± 0.05 and 0.65 ± 0.06 Hz (P < 0.001). Increasing the extracellular Ca²⁺ concentration enhanced this effect, with the frequency of spontaneous Ca²⁺ transients increasing from 0.45 ± 0.05 Hz to between 0.75 ± 0.06 and 0.94 ± 0.09 Hz after electrical stimulation at 3 to 9 Hz (P < 0.001), and this was accompanied by a significant increase in the velocity of Ca²⁺ transients that manifested as waves. Moreover, in the presence of high extracellular Ca²⁺, the spontaneous Ca²⁺ transients occurred more synchronously in the initial few seconds following electrical stimulation. The ryanodine receptors, which are the source of spontaneous Ca²⁺ transients in pulmonary vein cardiomyocytes, were found to be arranged in a striated pattern in the cell interior, as well as along the periphery of cell. Furthermore, labelling the sarcolemma with di-4-ANEPPS showed that over 90% of pulmonary vein cardiomyocytes possessed T-tubules. These findings demonstrate that the frequency of spontaneous Ca²⁺ transients in the rat pulmonary vein are increased following higher rates of electrical stimulation and increasing the extracellular Ca²⁺ concentration.

Intracellular dyssynchrony of diastolic cytosolic [Ca²⁺] decay in ventricular cardiomyocytes in cardiac remodeling and human heart failure

RATIONALE

Synchronized release of Ca²⁺ into the cytosol during each cardiac cycle determines cardiomyocyte contraction.

OBJECTIVE

We investigated synchrony of cytosolic [Ca²⁺] decay during diastole and the impact of cardiac remodeling.

METHODS AND RESULTS

Local cytosolic [Ca²⁺] transients (1-µm intervals) were recorded in murine, porcine, and human ventricular single cardiomyocytes. We identified intracellular regions of slow (slowCaR) and fast (fastCaR) [Ca²⁺] decay based on the local time constants of decay (TAUlocal). The SD of TAUlocal as a measure of dyssynchrony was not related to the amplitude or the timing of local Ca²⁺ release. Stimulation of sarcoplasmic reticulum Ca²⁺ ATPase with forskolin or istaroxime accelerated and its inhibition with cyclopiazonic acid slowed TAUlocal significantly more in slowCaR, thus altering the relationship between SD of TAUlocal and global [Ca²⁺] decay (TAUglobal). Na⁺/Ca²⁺ exchanger inhibitor SEA0400 prolonged TAUlocal similarly in slowCaR and fastCaR. FastCaR were associated with increased mitochondrial density and were more sensitive to the mitochondrial Ca²⁺ uniporter blocker Ru360. Variation in TAUlocal was higher in pig and human cardiomyocytes and higher with increased stimulation frequency (2 Hz). TAUlocal correlated with local sarcomere relengthening. In mice with myocardial hypertrophy after transverse aortic constriction, in pigs with chronic myocardial ischemia, and in end-stage human heart failure, variation in TAUlocal was increased and related to cardiomyocyte hypertrophy and increased mitochondrial density.

CONCLUSIONS

In cardiomyocytes, cytosolic [Ca²⁺] decay is regulated locally and related to local sarcomere relengthening. Dyssynchronous intracellular [Ca²⁺] decay in cardiac remodeling and end-stage heart failure suggests a novel mechanism of cellular contractile dysfunction.

Dyssynchrony of Ca2+ release from the sarcoplasmic reticulum as subcellular mechanism of cardiac contractile dysfunction

Cardiac contractile function depends on coordinated electrical activation throughout the heart. Dyssynchronous electrical activation of the ventricles has been shown to contribute to contractile dysfunction in heart failure, and resynchronization therapy has emerged as a therapeutic concept. At the cellular level, coupling of membrane excitation to myofilament contraction is facilitated by highly organized intracellular structures which coordinate Ca(2+) release. The cytosolic [Ca(2+)] transient triggered by depolarization-induced Ca(2+) influx is the result of a gradable and robust high gain process, Ca(2+)-induced Ca(2+) release (CICR), which integrates subcellular localized Ca(2+) release events. Lack of synchronization of these localized release events can contribute to contractile dysfunction in myocardial hypertrophy and heart failure. Different underlying mechanisms relate to functional and structural changes in sarcolemmal Ca(2+) channels, the sarcoplasmic Ca(2+) release channel or ryanodine receptor, RyR, their intracellular arrangement in close proximity in couplons and the loss of t-tubules. Dyssynchrony at the subcellular level translates in a reduction of the overall gain of CICR at the cellular level and forms an important determinant of myocyte contractility in heart failure.

Differential sensitivity of Ca²+ wave and Ca²+ spark events to ruthenium red in isolated permeabilised rabbit cardiomyocytes

Spontaneous Ca²(+) waves in cardiac muscle cells are thought to arise from the sequential firing of local Ca²(+) sparks via a fire-diffuse-fire mechanism. This study compares the ability of the ryanodine receptor (RyR) blocker ruthenium red (RuR) to inhibit these two types of Ca²(+) release in permeabilised rabbit ventricular cardiomyocytes. Perfusing with 600 nm Ca²(+) (50 μm EGTA) caused regular spontaneous Ca²(+) waves that were imaged with the fluorescence from Fluo-5F using a laser-scanning confocal microscope. Addition of 4 μm RuR caused complete inhibition of Ca²(+) waves in 50% of cardiomyocytes by 2 min and in 100% by 4 min. Separate experiments used 350 μm EGTA (600 nm Ca²(+)) to limit Ca²(+) diffusion but allow the underlying Ca(2+) sparks to be imaged. The time course of RuR-induced inhibition did not match that of waves. After 2 min of RuR, none of the characteristics of the Ca²(+) sparks were altered, and after 4 min Ca²(+) spark frequency was reduced ∼40%; no sparks could be detected after 10 min. Measurements of Ca(2+) within the SR lumen using Fluo-5N showed an increase in intra-SR Ca²(+) during the initial 2-4 min of perfusion with RuR in both wave and spark conditions. Computational modelling suggests that the sensitivity of Ca²(+) waves to RuR block depends on the number of RyRs per cluster. Therefore inhibition of Ca²(+) waves without affecting Ca²(+) sparks may be explained by block of small, non-spark producing clusters of RyRs that are important to the process of Ca²(+) wave propagation.

Measurement and modeling of Ca2+ waves in isolated rabbit ventricular cardiomyocytes

The time course and magnitude of the Ca(2+) fluxes underlying spontaneous Ca(2+) waves in single permeabilized ventricular cardiomyocytes were derived from confocal Fluo-5F fluorescence signals. Peak flux rates via the sarcoplasmic reticulum (SR) release channel (RyR2) and the SR Ca(2+) ATPase (SERCA) were not constant across a range of cellular [Ca(2+)] values. The Ca(2+) affinity (K(mf)) and maximum turnover rate (V(max)) of SERCA and the peak permeability of the RyR2-mediated Ca(2+) release pathway increased at higher cellular [Ca(2+)] loads. This information was used to create a computational model of the Ca(2+) wave, which predicted the time course and frequency dependence of Ca(2+) waves over a range of cellular Ca(2+) loads. Incubation of cardiomyocytes with the Ca(2+) calmodulin (CaM) kinase inhibitor autocamtide-2-related inhibitory peptide (300 nM, 30 mins) significantly reduced the frequency of the Ca(2+) waves at high Ca(2+) loads. Analysis of the Ca(2+) fluxes suggests that inhibition of CaM kinase prevented the increases in SERCA V(max) and peak RyR2 release flux observed at high cellular [Ca(2+)]. These data support the view that modification of activity of SERCA and RyR2 via a CaM kinase sensitive process occurs at higher cellular Ca(2+) loads to increase the maximum frequency of spontaneous Ca(2+) waves.