InsP3R-RyR Ca2+ channel crosstalk facilitates arrhythmias in the failing human ventricle

Dysregulated intracellular Ca²⁺ handling involving altered Ca²⁺ release from intracellular stores via RyR channels underlies both arrhythmias and reduced function in heart failure (HF). Mechanisms linking RyR dysregulation and disease are not fully established. Studies in animals support a role for InsP₃ receptor Ca²⁺ channels (InsP₃R) in pathological alterations in cardiomyocyte Ca²⁺ handling but whether these findings translate to the divergent physiology of human cardiomyocytes during heart failure is not determined. Using electrophysiological and Ca²⁺ recordings in human ventricular cardiomyocytes, we uncovered that Ca²⁺ release via InsP₃Rs facilitated Ca²⁺ release from RyR and induced arrhythmogenic delayed after depolarisations and action potentials. InsP₃R-RyR crosstalk was particularly increased in HF at RyR clusters isolated from the T-tubular network. Reduced SERCA activity in HF further facilitated the action of InsP₃. Nanoscale imaging revealed co-localisation of InsP₃Rs with RyRs in the dyad, which was increased in HF, providing a mechanism for augmented Ca²⁺ channel crosstalk. Notably, arrhythmogenic activity dependent on InsP₃Rs was increased in tissue wedges from failing hearts perfused with AngII to promote InsP₃ generation. These data indicate a central role for InsP₃R-RyR Ca²⁺ signalling crosstalk in the pro-arrhythmic action of GPCR agonists elevated in HF and the potential for their therapeutic targeting.

MSK-Mediated Phosphorylation of Histone H3 Ser28 Couples MAPK Signalling with Early Gene Induction and Cardiac Hypertrophy

Heart failure is a leading cause of death that develops subsequent to deleterious hypertrophic cardiac remodelling. MAPK pathways play a key role in coordinating the induction of gene expression during hypertrophy. Induction of the immediate early gene (IEG) response including activator protein 1 (AP-1) complex factors is a necessary and early event in this process. How MAPK and IEG expression are coupled during cardiac hypertrophy is not resolved. Here, in vitro, in rodent models and in human samples, we demonstrate that MAPK-stimulated IEG induction depends on the mitogen and stress-activated protein kinase (MSK) and its phosphorylation of histone H3 at serine 28 (pH3S28). pH3S28 in IEG promoters in turn recruits Brg1, a BAF60 ATP-dependent chromatin remodelling complex component, initiating gene expression. Without MSK activity and IEG induction, the hypertrophic response is suppressed. These studies provide new mechanistic insights into the role of MAPK pathways in signalling to the epigenome and regulation of gene expression during cardiac hypertrophy.

Myofibroblast Phenotype and Reversibility of Fibrosis in Patients With End-Stage Heart Failure

BACKGROUND

Interstitial fibrosis is an important component of diastolic, and systolic, dysfunction in heart failure (HF) and depends on activation and differentiation of fibroblasts into myofibroblasts (MyoFb). Recent clinical evidence suggests that in late-stage HF, fibrosis is not reversible.

OBJECTIVES

The study aims to examine the degree of differentiation of cardiac MyoFb in end-stage HF and the potential for their phenotypic reversibility.

METHODS

Fibroblasts were isolated from the left ventricle of the explanted hearts of transplant recipients (ischemic and dilated cardiomyopathy), and from nonused donor hearts. Fibroblasts were maintained in culture without passaging for 4 or 8 days (treatment studies). Phenotyping included functional testing, immunostaining, and expression studies for markers of differentiation. These data were complemented with immunohistology and expression studies in tissue samples.

RESULTS

Interstitial fibrosis with cross-linked collagen is prominent in HF hearts, with presence of activated MyoFbs. Tissue levels of transforming growth factor (TGF)-β1, lysyl oxidase, periostin, and osteopontin are elevated. Fibroblastic cells isolated from HF hearts are predominantly MyoFb, proliferative or nonproliferative, with mature α-smooth muscle actin stress fibers. HF MyoFb express high levels of profibrotic cytokines and the TGF-β1 pathway is activated. Inhibition of TGF-β1 receptor kinase in HF MyoFb promotes dedifferentiation of MyoFb with loss of α-smooth muscle actin and depolymerization of stress fibers, and reduces the expression of profibrotic genes and cytokines levels to non-HF levels.

CONCLUSION

MyoFb in end-stage HF have a variable degree of differentiation and retain the capacity to return to a less activated state, validating the potential for developing antifibrotic therapy targeting MyoFb.

Altered adrenergic response in myocytes bordering a chronic myocardial infarction underlies in vivo triggered activity and repolarization instability

Key points

Ventricular arrhythmias are a major complication after myocardial infarction (MI), associated with sympathetic activation. The structurally heterogeneous peri‐infarct zone is a known substrate, but the functional role of the myocytes is less well known.

Recordings of monophasic action potentials in vivo reveal that the peri‐infarct zone is a source of delayed afterdepolarizations (DADs) and has a high beat‐to‐beat variability of repolarization (BVR) during adrenergic stimulation (isoproterenol, ISO).

Myocytes isolated from the peri‐infarct region have more DADs and spontaneous action potentials, with spontaneous Ca²⁺ release, under ISO. These myocytes also have reduced repolarization reserve and increased BVR. Other properties of post‐MI remodelling are present in both peri‐infarct and remote myocytes.

These data highlight the importance of altered myocyte adrenergic responses in the peri‐infarct region as source and substrate of post‐MI arrhythmias.

Abstract

Ventricular arrhythmias are a major early complication after myocardial infarction (MI). The heterogeneous peri‐infarct zone forms a substrate for re‐entry while arrhythmia initiation is often associated with sympathetic activation. We studied the mechanisms triggering these post‐MI arrhythmias in vivo and their relation to regional myocyte remodelling. In pigs with chronic MI (6 weeks), in vivo monophasic action potentials were simultaneously recorded in the peri‐infarct and remote regions during adrenergic stimulation with isoproterenol (isoprenaline; ISO). Sham animals served as controls. During infusion of ISO in vivo, the incidence of delayed afterdepolarizations (DADs) and beat‐to‐beat variability of repolarization (BVR) was higher in the peri‐infarct than in the remote region. Myocytes isolated from the peri‐infarct region, in comparison to myocytes from the remote region, had more DADs, associated with spontaneous Ca²⁺ release, and a higher incidence of spontaneous action potentials (APs) when exposed to ISO (9.99 ± 4.2 vs. 0.16 ± 0.05 APs/min, p = 0.004); these were suppressed by CaMKII inhibition. Peri‐infarct myocytes also had reduced repolarization reserve and increased BVR (26 ± 10 ms vs. 9 ± 7 ms, P < 0.001), correlating with DAD activity. In contrast to these regional distinctions under ISO, alterations in Ca²⁺ handling at baseline and myocyte hypertrophy were present throughout the left ventricle (LV). Expression of some of the related genes was, however, different between the regions. In conclusion, altered myocyte adrenergic responses in the peri‐infarct but not the remote region provide a source of triggered activity in vivo and of repolarization instability amplifying the substrate for re‐entry. These findings stimulate further exploration of region‐specific therapies targeting myocytes and autonomic modulation.

Ventricular arrhythmias are a major complication after myocardial infarction (MI), associated with sympathetic activation. The structurally heterogeneous peri‐infarct zone is a known substrate, but the functional role of the myocytes is less well known.

Recordings of monophasic action potentials in vivo reveal that the peri‐infarct zone is a source of delayed afterdepolarizations (DADs) and has a high beat‐to‐beat variability of repolarization (BVR) during adrenergic stimulation (isoproterenol, ISO).

Myocytes isolated from the peri‐infarct region have more DADs and spontaneous action potentials, with spontaneous Ca²⁺ release, under ISO. These myocytes also have reduced repolarization reserve and increased BVR. Other properties of post‐MI remodelling are present in both peri‐infarct and remote myocytes.

These data highlight the importance of altered myocyte adrenergic responses in the peri‐infarct region as source and substrate of post‐MI arrhythmias.

Hyperactive ryanodine receptors in human heart failure and ischaemic cardiomyopathy reside outside of couplons

Aims

In ventricular myocytes from humans and large mammals, the transverse and axial tubular system (TATS) network is less extensive than in rodents with consequently a greater proportion of ryanodine receptors (RyRs) not coupled to this membrane system. TATS remodelling in heart failure (HF) and after myocardial infarction (MI) increases the fraction of non-coupled RyRs. Here we investigate whether this remodelling alters the activity of coupled and non-coupled RyR sub-populations through changes in local signalling. We study myocytes from patients with end-stage HF, compared with non-failing (non-HF), and myocytes from pigs with MI and reduced left ventricular (LV) function, compared with sham intervention (SHAM).

Methods and results

Single LV myocytes for functional studies were isolated according to standard protocols. Immunofluorescent staining visualized organization of TATS and RyRs. Ca²⁺ was measured by confocal imaging (fluo-4 as indicator) and using whole-cell patch-clamp (37°C). Spontaneous Ca²⁺ release events, Ca²⁺ sparks, as a readout for RyR activity were recorded during a 15 s period following conditioning stimulation at 2 Hz. Sparks were assigned to cell regions categorized as coupled or non-coupled sites according to a previously developed method. Human HF myocytes had more non-coupled sites and these had more spontaneous activity than in non-HF. Hyperactivity of these non-coupled RyRs was reduced by Ca²⁺/calmodulin-dependent kinase II (CaMKII) inhibition. Myocytes from MI pigs had similar changes compared with SHAM controls as seen in human HF myocytes. As well as by CaMKII inhibition, in MI, the increased activity of non-coupled sites was inhibited by mitochondrial reactive oxygen species (mito-ROS) scavenging. Under adrenergic stimulation, Ca²⁺ waves were more frequent and originated at non-coupled sites, generating larger Na⁺/Ca²⁺ exchange currents in MI than in SHAM. Inhibition of CaMKII or mito-ROS scavenging reduced spontaneous Ca²⁺ waves, and improved excitation–contraction coupling.

Conclusions

In HF and after MI, RyR microdomain re-organization enhances spontaneous Ca²⁺ release at non-coupled sites in a manner dependent on CaMKII activation and mito-ROS production. This specific modulation generates a substrate for arrhythmia that appears to be responsive to selective pharmacologic modulation.

Global fibroblast activation throughout the left ventricle but localized fibrosis after myocardial infarction

Fibroblast (Fb) differentiation and interstitial fibrosis contribute to cardiac remodeling and loss of function after myocardial infarction (MI). We investigated regional presence and regulation of fibrosis in a pig MI model. In vivo analysis of regional function and perfusion defined three regions: the scar, the myocardium adjacent to the scar (MI_adjacent, reduced function, reduced perfusion reserve), and the remote myocardium (MI_remote, minimal functional deficit, maintained perfusion). Interstitial and perivascular fibrosis, and increase of collagen type I, was only observed in the MI_adjacent. Fb activated protein-alpha (FAP-α) was enriched in MI_adjacent compared to MI_remote. TGF-β1, which triggers Fb differentiation, was upregulated in both MI_adjacent and MI_remote, whereas lysyl oxidase, a regulator of collagen cross-linking, and the proteoglycans decorin and biglycan were only increased in the MI_adjacent. Fb isolated and cultured for 4 days had myoFb characteristics with little difference between MI_remote and MI_adjacent, although RNA sequencing revealed differences in gene expression profiles. Fbs from all regions maintained proliferative capacity, and induced contraction of 3-D collagen matrices but scar myoFb was more effective. These data suggest that after MI, signaling through TGF-β1, possibly related to increased mechanical load, drives Fb activation throughout the left ventricle while regional signaling determines further maturation and extracellular matrix remodeling after MI.

Reduced mitochondrial respiration in the ischemic as well as in the remote nonischemic region in postmyocardial infarction remodeling

Scarring and remodeling of the left ventricle (LV) after myocardial infarction (MI) results in ischemic cardiomyopathy with reduced contractile function. Regional differences related to persisting ischemia may exist. We investigated the hypothesis that mitochondrial function and structure is altered in the myocardium adjacent to MI with reduced perfusion (MI_adjacent) and less so in the remote, nonischemic myocardium (MI_remote). We used a pig model of chronic coronary stenosis and MI (n = 13). Functional and perfusion MR imaging 6 wk after intervention showed reduced ejection fraction and increased global wall stress compared with sham-operated animals (Sham; n = 14). Regional strain in MI_adjacent was reduced with reduced contractile reserve; in MI_remote strain was also reduced but responsive to dobutamine and perfusion was normal compared with Sham. Capillary density was unchanged. Cardiac myocytes isolated from both regions had reduced basal and maximal oxygen consumption rate, as well as through complex I and II, but complex IV activity was unchanged. Reduced respiration was not associated with detectable reduction of mitochondrial density. There was no significant change in AMPK or glucose transporter expression levels, but glycogen content was significantly increased in both MI_adjacent and MI_remote Glycogen accumulation was predominantly perinuclear; mitochondria in this area were smaller but only in MI_adjacent where also subsarcolemmal mitochondria were smaller. In conclusion, after MI reduction of mitochondrial respiration and glycogen accumulation occur in all LV regions suggesting that reduced perfusion does not lead to additional specific changes and that increased hemodynamic load is the major driver for changes in mitochondrial function.

Reversible and irreversible differentiation of cardiac fibroblasts

AIMS

Differentiation of cardiac fibroblasts (Fbs) into myofibroblasts (MyoFbs) is responsible for connective tissue build-up in myocardial remodelling. We examined MyoFb differentiation and reversibility.

METHODS AND RESULTS

Adult rat cardiac Fbs were cultured on a plastic substratum providing mechanical stress, with conditions to obtain different levels of Fb differentiation. Fb spontaneously differentiated to proliferating MyoFb (p-MyoFb) with stress fibre formation decorated with alpha-smooth muscle actin (α-SMA). Transforming growth factor-β1 (TGF-β1) promoted differentiation into α-SMA-positive MyoFb showing near the absence of proliferation, i.e. non-p-MyoFb. SD-208, a TGF-β-receptor-I (TGF-β-RI) kinase blocker, inhibited p-MyoFb differentiation as shown by stress fibre absence, low α-SMA expression, and high proliferation levels. Fb seeded in collagen matrices induced no contraction, whereas p-MyoFb and non-p-MyoFb induced 2.5- and four-fold contraction. Fb produced little collagen but high levels of interleukin-10. Non-p-MyoFb had high collagen production and high monocyte chemoattractant protein-1 and tissue inhibitor of metalloproteinases-1 levels. Transcriptome analysis indicated differential activation of gene networks related to differentiation of MyoFb (e.g. paxilin and PAK) and reduced proliferation of non-p-MyoFb (e.g. cyclins and cell cycle regulation). Dedifferentiation of p-MyoFb with stress fibre de-polymerization, but not of non-p-MyoFb, was induced by SD-208 despite maintained stress. Stress fibre de-polymerization could also be induced by mechanical strain release in p-MyoFb and non-p-MyoFb (2-day cultures in unrestrained 3-D collagen matrices). Only p-MyoFb showed true dedifferentiation after long-term 3-D cultures.

CONCLUSIONS

Fb, p-MyoFb, and non-p-MyoFb have a distinct gene expression, ultrastructural, and functional profile. Both reduction in mechanical strain and TGF-β-RI kinase inhibition can reverse p-MyoFb differentiation but not non-p-MyoFb.

Glycogen synthase kinase-3α/β inhibition promotes in vivo amplification of endogenous mesenchymal progenitors with osteogenic and adipogenic potential and their differentiation to the osteogenic lineage

Small molecules are attractive therapeutics to amplify and direct differentiation of stem cells. They also can be used to understand the regulation of their fate by interfering with specific signaling pathways. Mesenchymal stem cells (MSCs) have the potential to proliferate and differentiate into several cell types, including osteoblasts. Activation of canonical Wnt signaling by inhibition of glycogen synthase kinase 3 (GSK-3) has been shown to enhance bone mass, possibly by involving a number of mechanisms ranging from amplification of the mesenchymal stem cell pool to the commitment and differentiation of osteoblasts. Here we have used a highly specific novel inhibitor of GSK-3, AR28, capable of inducing β-catenin nuclear translocation and enhanced bone mass after 14 days of treatment in BALB/c mice. We have shown a temporally regulated increase in the number of colony-forming units-osteoblast (CFU-O) and -adipocyte (CFU-A) but not colony-forming units-fibroblast (CFU-F) in mice treated for 3 days. However, the number of CFU-O and CFU-A returned to normal levels after 14 days of treatment, and the number of CFU-F was decreased significantly. In contrast, the number of osteoblasts increased significantly only after 14 days of treatment, and this was seen together with a significant decrease in bone marrow adiposity. These data suggest that the increased bone mass is the result of an early temporal wave of amplification of a subpopulation of MSCs with both osteogenic and adipogenic potential, which is driven to osteoblast differentiation at the expense of adipogenesis.