Regulation of cardiomyocyte t-tubule structure by preload and afterload: Roles in cardiac compensation and decompensation.

Mechanical load is a potent regulator of cardiac structure and function. Although high workload during heart failure is associated with disruption of cardiomyocyte t-tubules and Ca2+ homeostasis, it remains unclear whether changes in preload and afterload may promote adaptive t-tubule remodelling. We examined this issue by first investigating isolated effects of stepwise increases in load in cultured rat papillary muscles. Both preload and afterload increases produced a biphasic response, with the highest t-tubule densities observed at moderate loads, whereas excessively low and high loads resulted in low t-tubule levels. To determine the baseline position of the heart on this bell-shaped curve, mice were subjected to mildly elevated preload or afterload (1 week of aortic shunt or banding). Both interventions resulted in compensated cardiac function linked to increased t-tubule density, consistent with ascension up the rising limb of the curve. Similar t-tubule proliferation was observed in human patients with moderately increased preload or afterload (mitral valve regurgitation, aortic stenosis). T-tubule growth was associated with larger Ca2+ transients, linked to upregulation of L-type Ca2+ channels, Na+-Ca2+ exchanger, mechanosensors and regulators of t-tubule structure. By contrast, marked elevation of cardiac load in rodents and patients advanced the heart down the declining limb of the t-tubule-load relationship. This bell-shaped relationship was lost in the absence of electrical stimulation, indicating a key role of systolic stress in controlling t-tubule plasticity. In conclusion, modest augmentation of workload promotes compensatory increases in t-tubule density and Ca2+ cycling, whereas this adaptation is reversed in overloaded hearts during heart failure progression. KEY POINTS: Excised papillary muscle experiments demonstrated a bell-shaped relationship between cardiomyocyte t-tubule density and workload (preload or afterload), which was only present when muscles were electrically stimulated. The in vivo heart at baseline is positioned on the rising phase of this curve because moderate increases in preload (mice with brief aortic shunt surgery, patients with mitral valve regurgitation) resulted in t-tubule growth. Moderate increases in afterload (mice and patients with mild aortic banding/stenosis) similarly increased t-tubule density. T-tubule proliferation was associated with larger Ca2+ transients, with upregulation of the L-type Ca2+ channel, Na+-Ca2+ exchanger, mechanosensors and regulators of t-tubule structure. By contrast, marked elevation of cardiac load in rodents and patients placed the heart on the declining phase of the t-tubule-load relationship, promoting heart failure progression. The dependence of t-tubule structure on preload and afterload thus enables both compensatory and maladaptive remodelling, in rodents and humans.

Regulation of titin-based cardiac stiffness by unfolded domain oxidation (UnDOx).

The relationship between oxidative stress and cardiac stiffness is thought to involve modifications to the giant muscle protein titin, which in turn can determine the progression of heart disease. In vitro studies have shown that S-glutathionylation and disulfide bonding of titin fragments could alter the elastic properties of titin; however, whether and where titin becomes oxidized in vivo is less certain. Here we demonstrate, using multiple models of oxidative stress in conjunction with mechanical loading, that immunoglobulin domains preferentially from the distal titin spring region become oxidized in vivo through the mechanism of unfolded domain oxidation (UnDOx). Via oxidation type-specific modification of titin, UnDOx modulates human cardiomyocyte passive force bidirectionally. UnDOx also enhances titin phosphorylation and, importantly, promotes nonconstitutive folding and aggregation of unfolded domains. We propose a mechanism whereby UnDOx enables the controlled homotypic interactions within the distal titin spring to stabilize this segment and regulate myocardial passive stiffness.

DNA Methylation Analysis Identifies Novel Epigenetic Loci in Dilated Murine Heart upon Exposure to Volume Overload.

Left ventricular (LV) dilatation, a prominent risk factor for heart failure (HF), precedes functional deterioration and is used to stratify patients at risk for arrhythmias and cardiac mortality. Aberrant DNA methylation contributes to maladaptive cardiac remodeling and HF progression following pressure overload and ischemic cardiac insults. However, no study has examined cardiac DNA methylation upon exposure to volume overload (VO) despite being relatively common among HF patients. We carried out global methylome analysis of LV harvested at a decompensated HF stage following exposure to VO induced by aortocaval shunt. VO resulted in pathological cardiac remodeling, characterized by massive LV dilatation and contractile dysfunction at 16 weeks after shunt. Although methylated DNA was not markedly altered globally, 25 differentially methylated promoter regions (DMRs) were identified in shunt vs. sham hearts (20 hypermethylated and 5 hypomethylated regions). The validated hypermethylated loci in Junctophilin-2 (Jph2), Signal peptidase complex subunit 3 (Spcs3), Vesicle-associated membrane protein-associated protein B (Vapb), and Inositol polyphosphate multikinase (Ipmk) were associated with the respective downregulated expression and were consistently observed in dilated LV early after shunt at 1 week after shunt, before functional deterioration starts to manifest. These hypermethylated loci were also detected peripherally in the blood of the shunt mice. Altogether, we have identified conserved DMRs that could be novel epigenetic biomarkers in dilated LV upon VO exposure.

Hemodynamic stress-induced cardiac remodelling is not modulated by ablation of phosphodiesterase 4D interacting protein.

Adrenergic stimulation in the heart activates the protein kinase A (PKA), which phosphorylates key proteins involved in intracellular Ca2+ handling. PKA is held in proximity to its substrates by protein scaffolds, the A kinase anchoring proteins (AKAPs). We have previously identified the transcript of phosphodiesterase 4D interacting protein (Pde4dip; also known as myomegalin), one of the sarcomeric AKAPs, as being differentially expressed following hemodynamic overload, a condition inducing hyperadrenergic state in the heart. Here, we addressed whether PDE4DIP is involved in the adverse cardiac remodelling following hemodynamic stress. Homozygous Pde4dip knockout (KO) mice, generated by CRISPR-Cas9 technology, and wild-type (WT) littermates were exposed to aortocaval shunt (shunt) or transthoracic aortic constriction (TAC) to induce hemodynamic volume overload (VO) or pressure overload (PO), respectively. The mortality, cardiac structure, function and pathological cardiac remodelling were followed up after hemodynamic injuries. The PDE4DIP protein level was markedly downregulated in volume-overloaded- but upregulated in pressure-overloaded-WT hearts. Following shunt or TAC, mortality rates were comparably increased in both genotypes. Twelve weeks after shunt or TAC, Pde4dip-KO animals showed a similar degree of cardiac hypertrophy, dilatation and dysfunction as WT mice. Cardiomyocyte hypertrophy, myocardial fibrosis, reactivation of cardiac stress genes and downregulation of ATPase, Ca2+ transporting, cardiac muscle, slow twitch 2 transcript did not differ between WT and Pde4dip-KO hearts following shunt or TAC. In summary, despite a differential expression of PDE4DIP protein in remodelled WT hearts, Pde4dip deficiency does not modulate adverse cardiac remodelling after hemodynamic VO or PO.

Heat shock protein A4 ablation leads to skeletal muscle myopathy associated with dysregulated autophagy and induced apoptosis.

BACKGROUND: Molecular chaperones assist protein folding, facilitate degradation of misfolded polypeptides, and thereby maintain protein homeostasis. Impaired chaperone activity leads to defective protein quality control that is implicated in multiple skeletal muscle diseases. The heat shock protein A4 (HSPA4) acts as a co-chaperone for HSP70. Previously, we showed that Hspa4 deletion causes impaired protein homeostasis in the heart. However, its functional role in skeletal muscle has not been explored. METHODS: We performed a comparative phenotypic and biochemical analyses of Hspa4 knockout (KO) mice with wild-type (WT) littermates. RESULTS: HSPA4 is markedly upregulated in regenerating WT muscle in vivo, and in differentiated myoblasts in vitro. Hspa4-KO mice are marked by growth retardation and increased variability in body weight, accompanied by 35% mortality rates during the peri-weaning period. The surviving Hspa4-KO mice experienced progressive skeletal muscle myopathy, characterized by increased number of muscle fibers with centralized nuclei, heterogeneous myofiber size distribution, inflammatory cell infiltrates and upregulation of embryonic and perinatal myosin heavy chain transcripts. Hspa4-KO muscles demonstrated an accumulation of autophagosome-associated proteins including microtubule associated protein1 light chain 3-II (LC3-II) and p62/sequestosome accompanied by increased number of TUNEL-positive nuclei. CONCLUSIONS: Our findings underscore the indispensable role of HSPA4 in maintenance of muscle integrity through contribution in skeletal muscle autophagy and apoptosis, which might provide a novel therapeutic strategy for skeletal muscle morbidities.

CRISPLD1: a novel conserved target in the transition to human heart failure.

Heart failure is a major health problem worldwide with a significant morbidity and mortality rate. Although studied extensively in animal models, data from patients at the compensated disease stage are lacking. We sampled myocardium biopsies from aortic stenosis patients with compensated hypertrophy and moderate heart failure and used transcriptomics to study the transition to failure. Sequencing and comparative analysis of analogous samples of mice with transverse aortic constriction identified 25 candidate genes with similar regulation in response to pressure overload, reflecting highly conserved molecular processes. The gene cysteine-rich secretory protein LCCL domain containing 1 (CRISPLD1) is upregulated in the transition to failure in human and mouse and its function is unknown. Homology to ion channel regulatory toxins suggests a role in Ca2+ cycling. CRISPR/Cas9-mediated loss-of-function leads to dysregulated Ca2+ handling in human-induced pluripotent stem cell-derived cardiomyocytes. The downregulation of prohypertrophic, proapoptotic and Ca2+-signaling pathways upon CRISPLD1-KO and its upregulation in the transition to failure implicates a contribution to adverse remodeling. These findings provide new pathophysiological data on Ca2+ regulation in the transition to failure and novel candidate genes with promising potential for therapeutic interventions.

Proteomic analysis of short-term preload-induced eccentric cardiac hypertrophy.

BACKGROUND: Hemodynamic load leads to cardiac hypertrophy and heart failure. While afterload (pressure overload) induces concentric hypertrophy, elevation of preload (volume overload) yields eccentric hypertrophy and is associated with a better outcome. Here we analysed the proteomic pattern of mice subjected to short-term preload. METHODS AND RESULTS: Female FVB/N mice were subjected to aortocaval shunt-induced volume overload that leads to an eccentric hypertrophy (left ventricular weight/tibia length +31 %) with sustained systolic heart function at 1 week after operation. Two-dimensional gel electrophoresis (2-DE) followed by mass spectrometric analysis showed alteration in the expression of 25 protein spots representing 21 different proteins. 64 % of these protein spots were up-regulated and 36 % of the protein spots were consistently down-regulated. Interestingly, α-1-antitrypsin was down-regulated, indicating higher elastin degradation and possibly contributing to the early dilatation. In addition to contractile and mitochondrial proteins, polymerase I and transcript release factor protein (PTRF) was also up-regulated, possibly contributing to the preload-induced signal transduction. CONCLUSIONS: Our findings reveal the proteomic changes of early-stage eccentric myocardial remodeling after volume overload. Induced expression of some of the respiratory chain enzymes suggests a metabolic shift towards an oxidative phosphorylation that might contribute to the favorable remodeling seen in early VO. Down-regulation of α-1-antitrypsin might contribute to extracellular matrix remodeling and left ventricular dilatation. We also identified PTRF as a potential signaling regulator of volume overload-induced cardiac hypertrophy.

Respiratory distress and early neonatal lethality in Hspa4l/Hspa4 double-mutant mice.

Heat shock proteins HSPA4L and HSPA4 are closely related members of the HSP110 family and act as cochaperones. We generated Hspa4l(-/-)Hspa4(-/-) mice to investigate a functional complementarity between HSPA4L and HSPA4 during embryonic development. Hspa4l(-/-)Hspa4(-/-) embryos exhibited marked pulmonary hypoplasia and neonatal death. Compared with lungs of wild-type, Hspa4l(-/-), and Hspa4(-/-) embryos, Hspa4l(-/-)Hspa4(-/-) lungs were characterized by diminished saccular spaces and increased mesenchymal septa. Mesenchymal hypercellularity was determined to be due to an increased cell proliferation index and decreased cell death. A significant increase in expression levels of prosurvival protein B cell leukemia/lymphoma 2 may be the cause for inhibition of apoptotic process in lungs of Hspa4(-/-)Hspa4l(-/-) embryos. Accumulation of glycogen and diminished expression of surfactant protein B, prosurfactant protein C, and aquaporin 5 in saccular epithelium suggested impaired maturation of type II and type I pneumocytes in the Hspa4l(-/-)Hspa4(-/-) lungs. Further experiments showed a significant accumulation of ubiquitinated proteins in the lungs of Hspa4l(-/-)Hspa4(-/-) embryos, indicating an impaired chaperone activity. Our study demonstrates that HSPA4L and HSPA4 collaborate in embryonic lung maturation, which is necessary for adaptation to air breathing at birth.

A suspension of inactivated bacteria used for vaccination against recurrent urinary tract infections increases the phagocytic activity of murine macrophages.

Background: Urinary tract infections are a major cause of the consumption of antibiotics in humans. Methods: We studied the effect of a vaccine (StroVac®, containing inactivated bacteria and used to prevent recurrent urinary tract infections) licensed in Germany on the release of pro-inflammatory cytokines and the phagocytosis of Escherichia (E.) coli in primary murine macrophages and the macrophage cell line J774A.1. Results: StroVac® increased the release of the cytokines TNF-α, IL-6, IL-12/23 p40, and IL-1ß and stimulated the phagocytosis of E. coli in a dose-dependent manner. This effect was independent of LPS as shown by the use of macrophages isolated from LPS-resistant C3H/HeJ mice. At concentrations up to 30 mg/l it was not toxic to bacteria or eukaryotic cells. Conclusion: StroVac® does not only act via the adaptive but also by stimulating the innate immune system. This stimulation may help to build trained innate immunity against bacterial pathogens involved in recurrent urinary tract infections.

Metabolomic Profiling in Patients with Different Hemodynamic Subtypes of Severe Aortic Valve Stenosis.

Severe aortic stenosis (AS) is a common pathological condition in an ageing population imposing significant morbidity and mortality. Based on distinct hemodynamic features, i.e., ejection fraction (EF), transvalvular gradient and stroke volume, four different AS subtypes can be distinguished: (i) normal EF and high gradient, (ii) reduced EF and high gradient, (iii) reduced EF and low gradient, and (iv) normal EF and low gradient. These subtypes differ with respect to pathophysiological mechanisms, cardiac remodeling, and prognosis. However, little is known about metabolic changes in these different hemodynamic conditions of AS. Thus, we carried out metabolomic analyses in serum samples of 40 AS patients (n = 10 per subtype) and 10 healthy blood donors (controls) using ultrahigh-performance liquid chromatography-tandem mass spectroscopy. A total of 1293 biochemicals could be identified. Principal component analysis revealed different metabolic profiles in all of the subgroups of AS (All-AS) vs. controls. Out of the determined biochemicals, 48% (n = 620) were altered in All-AS vs. controls (p < 0.05). In this regard, levels of various acylcarnitines (e.g., myristoylcarnitine, fold-change 1.85, p < 0.05), ketone bodies (e.g., 3-hydroxybutyrate, fold-change 11.14, p < 0.05) as well as sugar metabolites (e.g., glucose, fold-change 1.22, p < 0.05) were predominantly increased, whereas amino acids (e.g., leucine, fold-change 0.8, p < 0.05) were mainly reduced in All-AS. Interestingly, these changes appeared to be consistent amongst all AS subtypes. Distinct differences between AS subtypes were found for metabolites belonging to hemoglobin metabolism, diacylglycerols, and dihydrosphingomyelins. These findings indicate that relevant changes in substrate utilization appear to be consistent for different hemodynamic subtypes of AS and may therefore reflect common mechanisms during AS-induced heart failure. Additionally, distinct metabolites could be identified to significantly differ between certain AS subtypes. Future studies need to define their pathophysiological implications.

Targeted disruption of Hspa4 gene leads to cardiac hypertrophy and fibrosis.

Failure of molecular chaperones to direct the correct folding of newly synthesized proteins leads to the accumulation of misfolded proteins in cells. HSPA4 is a member of the heat shock protein 110 family (HSP110) that acts as a nucleotide exchange factor of HSP70 chaperones. We found that the expression of HSPA4 is upregulated in murine hearts subjected to pressure overload and in failing human hearts. To investigate the cardiac function of HSPA4, Hspa4 knockout (KO) mice were generated and exhibited cardiac hypertrophy and fibrosis. Hspa4 KO hearts were characterized by a significant increase in heart weight/body weight ratio, elevated expression of hypertrophic and fibrotic gene markers, and concentric hypertrophy with preserved contractile function. In response to pressure overload, cardiac hypertrophy and remodeling were further aggravated in the Hspa4 KO compared to wild type (WT) mice. Cardiac hypertrophy in Hspa4 KO hearts was associated with enhanced activation of gp130-STAT3, CaMKII, and calcineurin-NFAT signaling. Protein blot and immunofluorescent analyses showed a significant accumulation of polyubiquitinated proteins in cardiac cells of Hspa4 KO mice. These results suggest that the myocardial remodeling of Hspa4 KO mice is due to accumulation of misfolded proteins resulting from impaired chaperone activity. Further analyses revealed a significant increase in cross sectional area of cardiomyocytes, and in expression levels of hypertrophic markers in cultured neonatal Hspa4 KO cardiomyocytes suggesting that the hypertrophy of mutant mice was a result of primary defects in cardiomyocytes. Gene expression profile in hearts of 3.5-week-old mice revealed a differentially expressed gene sets related to ion channels, muscle-specific contractile proteins and stress response. Taken together, our in vivo data demonstrate that Hspa4 gene ablation results in cardiac hypertrophy and fibrosis, possibly, through its role in protein quality control mechanism.

Murine Embryonic Stem Cell Culture, Self-Renewal, and Differentiation.

Embryonic stem cells (ESCs), derived from the inner cell mass of the blastocyst, can proliferate indefinitely in vitro (self-renewal) and can differentiate into cells of all three germ layers (pluripotency). These unique properties make them exceptionally valuable in basic science and clinical researches, including cell replacement therapies, drug discovery, and regenerative medicine. Mouse ESCs represent an important model system for studying gene function during development and disease.ESCs culture is time-consuming, laborious, and costly. Suboptimal ESCs culture conditions can alter their identity, pluripotency, and their compatibility with downstream differentiation protocols. In this chapter, we provide a general guideline for murine ESCs culture on murine fibroblast feeder layers. Moreover, we describe protocols for maintenance of ESCs pluripotency and induction of ESCs differentiation.

Different activation of MAPKs and Akt/GSK3ß after preload vs. afterload elevation.

AIMS: Pressure overload (PO) and volume overload (VO) lead to concentric or eccentric hypertrophy. Previously, we could show that activation of signalling cascades differ in in vivo mouse models. Activation of these signal cascades could either be induced by intrinsic load sensing or neuro-endocrine substances like catecholamines or the renin-angiotensin-aldosterone system. METHODS AND RESULTS: We therefore analysed the activation of classical cardiac signal pathways [mitogen-activated protein kinases (MAPKs) (ERK, p38, and JNK) and Akt-GSK3ß] in in vitro of mechanical overload (ejecting heart model, rabbit and human isolated muscle strips). Selective elevation of preload in vitro increased AKT and GSK3ß phosphorylation after 15 min in isolated rabbit muscles strips (AKT 49%, GSK3ß 26%, P < 0.05) and in mouse ejecting hearts (AKT 51%, GSK49%, P < 0.05), whereas phosphorylation of MAPKs was not influenced by increased preload. Selective elevation of afterload revealed an increase in ERK phosphorylation in the ejecting heart (43%, P < 0.05), but not in AKT, GSK3ß, and the other MAPKs. Elevation of preload and afterload in the ejecting heart induced a significant phosphorylation of ERK (95%, P < 0.001) and showed a moderate increased AKT (P = 0.14) and GSK3ß (P = 0.21) phosphorylation, which did not reach significance. Preload and afterload elevation in muscles strips from human failing hearts showed neither AKT nor ERK phosphorylation changes. CONCLUSIONS: Our data show that preload activates the AKT-GSK3ß and afterload the ERK pathway in vitro, indicating an intrinsic mechanism independent of endocrine signalling.

Heat-shock protein HSPA4 is required for progression of spermatogenesis.

Heat-shock protein 110 (HSP110) family members act as nucleotide exchange factors (NEF) of mammalian and yeast HSP70 chaperones during the ATP hydrolysis cycle. In this study, we describe the expression pattern of murine HSPA4, a member of the HSP110 family, during testis development and the consequence of HSPA4 deficiency on male fertility. HSPA4 is ubiquitously expressed in all the examined tissues. During prenatal and postnatal development of gonad, HSPA4 is expressed in both somatic and germ cells; however, expression was much higher in germ cells of prenatal gonads. Analyses of Hspa4-deficient mice revealed that all homozygous mice on the hybrid C57BL/6J×129/Sv genetic background were apparently healthy. Although HSPA4 is expressed as early as E13.5 in male gonad, a lack of histological differences between Hspa4(-/-) and control littermates suggests that Hspa4 deficiency does not impair the gonocytes or their development to spermatogonia. Remarkably, an increased number of the Hspa4-deficient males displayed impaired fertility, whereas females were fertile. The total number of spermatozoa and their motility were drastically reduced in infertile Hspa4-deficient mice compared with wild-type littermates. The majority of pachytene spermatocytes in the juvenile Hspa4(-/-) mice failed to complete the first meiotic prophase and became apoptotic. Furthermore, down-regulation of transcription levels of genes known to be expressed in spermatocytes at late stages of prophase I and post-meiotic spermatids leads to suggest that the development of most spermatogenic cells is arrested at late stages of meiotic prophase I. These results provide evidence that HSPA4 is required for normal spermatogenesis.

Perturbed differentiation of murine embryonic stem cells upon Pelota deletion due to dysregulated FOXO1/ß-catenin signaling.

Differentiation of the embryonic stem cells (ESCs) is regulated by a variety of different signaling pathways. Genetic depletion of murine Pelota gene (Pelo) leads to early embryonic lethality. Here, we aimed at determining the embryonic stage and deciphering the dysregulated signaling pathways affected upon Pelo deletion. We found that development of PELO-null embryos is perturbed between the embryonic days E4.5 and E5.5, at which first differentiation process of ESCs takes place. Molecular analysis revealed enhanced activity of phosphoinositide 3-kinase-protein kinase B/ AKT (PI3K-PKB/AKT) signaling, but nuclear accumulation of forkhead box O1 (FOXO1), and upregulation of the pluripotency-related gene, Oct4, in mutant ESCs cultured under differentiation condition. Despite increased levels of nuclear ß-catenin in PELO-null ESCs as a result of decreased activity of glycogen synthase kinase-3ß, the activity of the canonical wingless (Wnt)/ß-catenin/T-cell factor (TCF) was significantly attenuated as judged by the promoter reporter assay, downregulated Wnt/ß-catenin target genes, and impaired cell proliferation. Interestingly, we demonstrated an increased binding of ß-catenin to FOXO1 in PELO-mutant ESCs cultured under differentiation condition that could explain, on one side, the nuclear accumulation of FOXO1 protein and hence persistent pluripotency of PELO-mutant ESCs, and on the other side, the dysregulated transcriptional activity of ß-catenin/TCF and therefore attenuated PELO-null ESC self-renewal. Taken together, our results strongly suggest that PELO deletion averts ESC differentiation through promoting FOXO1/ß-catenin binding with subsequent dysregulation of FOXO1 and canonical ß-catenin/TCF signaling pathways.

NADPH oxidase-4 promotes eccentric cardiac hypertrophy in response to volume overload.

AIMS: Chronic pressure or volume overload induce concentric vs. eccentric left ventricular (LV) remodelling, respectively. Previous studies suggest that distinct signalling pathways are involved in these responses. NADPH oxidase-4 (Nox4) is a reactive oxygen species-generating enzyme that can limit detrimental cardiac remodelling in response to pressure overload. This study aimed to assess its role in volume overload-induced remodelling. METHODS AND RESULTS: We compared the responses to creation of an aortocaval fistula (Shunt) to induce volume overload in Nox4-null mice (Nox4-/-) vs. wild-type (WT) littermates. Induction of Shunt resulted in a significant increase in cardiac Nox4 mRNA and protein levels in WT mice as compared to Sham controls. Nox4-/- mice developed less eccentric LV remodelling than WT mice (echocardiographic relative wall thickness: 0.30 vs. 0.27, P < 0.05), with less LV hypertrophy at organ level (increase in LV weight/tibia length ratio of 25% vs. 43%, P < 0.01) and cellular level (cardiomyocyte cross-sectional area: 323 µm2 vs. 379 µm2, P < 0.01). LV ejection fraction, foetal gene expression, interstitial fibrosis, myocardial capillary density, and levels of myocyte apoptosis after Shunt were similar in the two genotypes. Myocardial phospho-Akt levels were increased after induction of Shunt in WT mice, whereas levels decreased in Nox4-/- mice (+29% vs. -21%, P < 0.05), associated with a higher level of phosphorylation of the S6 ribosomal protein (S6) and the eIF4E-binding protein 1 (4E-BP1) in WT compared to Nox4-/- mice. We identified that Akt activation in cardiac cells is augmented by Nox4 via a Src kinase-dependent inactivation of protein phosphatase 2A. CONCLUSION: Endogenous Nox4 is required for the full development of eccentric cardiac hypertrophy and remodelling during chronic volume overload. Nox4-dependent activation of Akt and its downstream targets S6 and 4E-BP1 may be involved in this effect.

X-ray diffraction and second harmonic imaging reveal new insights into structural alterations caused by pressure-overload in murine hearts.

We demonstrate a label-free imaging approach to study cardiac remodeling of fibrotic and hypertrophic hearts, bridging scales from the whole organ down to the molecular level. To this end, we have used mice subjected to transverse aortic constriction and imaged adjacent cardiac tissue sections by microfocus X-ray diffraction and second harmonic generation (SHG) imaging. In this way, the acto-myosin structure was probed in a spatially resolved manner for entire heart sections. From the recorded diffraction data, spatial maps of diffraction intensity, anisotropy and orientation were obtained, and fully automated analysis depicted the acto-myosin filament spacing and direction. X-ray diffraction presented an overview of entire heart sections and revealed that in regions of severe cardiac remodeling the muscle mass is partly replaced by connective tissue and the acto-myosin lattice spacing is increased at these regions. SHG imaging revealed sub-cellular structure of cardiac tissue and complemented the findings from X-ray diffraction by revealing micro-level distortion of myofibrils, immune cell infiltration at regions of cardiac remodeling and the development of fibrosis down to the scale of a single collagen fibril. Overall, our results show that both X-ray diffraction and SHG imaging can be used for label-free and high-resolution visualization of cardiac remodeling and fibrosis progression at different stages in a cardiac pressure-overload mouse model that cannot be achieved by conventional histology.

Changes in m6A RNA methylation contribute to heart failure progression by modulating translation.

AIMS: Deregulation of epigenetic processes and aberrant gene expression are important mechanisms in heart failure. Here we studied the potential relevance of m6A RNA methylation in heart failure development. METHODS AND RESULTS: We analysed m6A RNA methylation via next-generation sequencing. We found that approximately one quarter of the transcripts in the healthy mouse and human heart exhibit m6A RNA methylation. During progression to heart failure we observed that changes in m6A RNA methylation exceed changes in gene expression both in mouse and human. RNAs with altered m6A RNA methylation were mainly linked to metabolic and regulatory pathways, while changes in RNA expression level mainly represented changes in structural plasticity. Mechanistically, we could link m6A RNA methylation to altered RNA translation and protein production. Interestingly, differentially methylated but not differentially expressed RNAs showed differential polysomal occupancy, indicating transcription-independent modulation of translation. Furthermore, mice with a cardiomyocyte restricted knockout of the RNA demethylase Fto exhibited an impaired cardiac function compared to control mice. CONCLUSIONS: We could show that m6A landscape is altered in heart hypertrophy and heart failure. m6A RNA methylation changes lead to changes in protein abundance, unconnected to mRNA levels. This uncovers a new transcription-independent mechanisms of translation regulation. Therefore, our data suggest that modulation of epitranscriptomic processes such as m6A methylation might be an interesting target for therapeutic interventions.

Genetic deletion of calcium/calmodulin-dependent protein kinase type II delta does not mitigate adverse myocardial remodeling in volume-overloaded hearts.

Calcium/calmodulin-dependent protein kinase type II delta (CaMKIIδ), the predominant CaMKII isoform expressed in the heart, has been implicated in the progression of myocardial infarction- and pressure overload-induced pathological remodeling. However, the role of CaMKIIδ in volume overload (VO) has not been explored. We have previously reported an activation of CaMKII during transition to HF in long-term VO. Here, we address whether CaMKIIδ is critically involved in the mortality, myocardial remodeling, and heart failure (HF) progression in response to VO. CaMKIIδ knockout (δ-KO) and wild-type (WT) littermates were exposed to aortocaval shunt-induced VO, and the progression of adverse myocardial remodeling was assessed by serial echocardiography, histological and molecular analyses. The mortality rates during 10 weeks of VO were similar in δ-KO and WT mice. Both genotypes displayed comparable eccentric myocardial hypertrophy, altered left ventricle geometry, perturbed systolic and diastolic functions after shunt. Additionally, cardiomyocytes hypertrophy, augmented myocyte apoptosis, and up-regulation of hypertrophic genes were also not significantly different in δ-KO versus WT hearts after shunt. Therefore, CaMKIIδ signaling seems to be dispensable for the progression of VO-induced maladaptive cardiac remodeling. Accordingly, we hypothesize that CaMKIIδ-inhibition as a therapeutic approach might not be helpful in the context of VO-triggered HF.

Assessing the role of extracellular signal-regulated kinases 1 and 2 in volume overload-induced cardiac remodelling.

AIMS: Volume overload (VO) and pressure overload (PO) induce differential cardiac remodelling responses including distinct signalling pathways. Extracellular signal-regulated kinases 1 and 2 (ERK1/2), key signalling components in the mitogen-activated protein kinase (MAPK) pathways, modulate cardiac remodelling during pressure overload (PO). This study aimed to assess their role in VO-induced cardiac remodelling as this was unknown. METHODS AND RESULTS: Aortocaval fistula (Shunt) surgery was performed in mice to induce cardiac VO. Two weeks of Shunt caused a significant reduction of cardiac ERK1/2 activation in wild type (WT) mice as indicated by decreased phosphorylation of the TEY (Thr-Glu-Tyr) motif (-28% as compared with Sham controls, P < 0.05). Phosphorylation of other MAPKs was unaffected. For further assessment, transgenic mice with cardiomyocyte-specific ERK2 overexpression (ERK2tg) were studied. At baseline, cardiac ERK1/2 phosphorylation in ERK2tg mice remained unchanged compared with WT littermates, and no overt cardiac phenotype was observed; however, cardiac expression of the atrial natriuretic peptide was increased on messenger RNA (3.6-fold, P < 0.05) and protein level (3.1-fold, P < 0.05). Following Shunt, left ventricular dilation and hypertrophy were similar in ERK2tg mice and WT littermates. Left ventricular function was maintained, and changes in gene expression indicated reactivation of the foetal gene program in both genotypes. No differences in cardiac fibrosis and kinase activation was found amongst all experimental groups, whereas apoptosis was similarly increased through Shunt in ERK2tg and WT mice. CONCLUSIONS: VO-induced eccentric hypertrophy is associated with reduced cardiac ERK1/2 activation in vivo. Cardiomyocyte-specific overexpression of ERK2, however, does not alter cardiac remodelling during VO. Future studies need to define the pathophysiological relevance of decreased ERK1/2 signalling during VO.