Chromatin remodelling precedes transcriptional and structural changes in heart failure. A body of work suggests roles for the developmental Wnt signalling pathway in cardiac remodelling. Hitherto, there is no evidence supporting a direct role of Wnt nuclear components in regulating chromatin landscapes in this process. We show that transcriptionally active, nuclear, phosphorylated(p)Ser675-β-catenin and TCF7L2 are upregulated in diseased murine and human cardiac ventricles. We report that inducible cardiomyocytes (CM)-specific pSer675-β-catenin accumulation mimics the disease situation by triggering TCF7L2 expression. This enhances active chromatin, characterized by increased H3K27ac and TCF7L2 occupancies to cardiac developmental and remodelling genes in vivo. Accordingly, transcriptomic analysis of β-catenin stabilized hearts shows a strong recapitulation of cardiac developmental processes like cell cycling and cytoskeletal remodelling. Mechanistically, TCF7L2 co-occupies distal genomic regions with cardiac transcription factors NKX2–5 and GATA4 in stabilized-β-catenin hearts. Validation assays revealed a previously unrecognized function of GATA4 as a cardiac repressor of the TCF7L2/β-catenin complex in vivo, thereby defining a transcriptional switch controlling disease progression. Conversely, preventing β-catenin activation post-pressure-overload results in a downregulation of these novel TCF7L2-targets and rescues cardiac function. Thus, we present a novel role for TCF7L2/β-catenin in CMs-specific chromatin modulation, which could be exploited for manipulating the ubiquitous Wnt pathway.

Rationale: Genome editing by CRISPR (clustered regularly interspaced short palindromic repeats)/Cas9 is evolving rapidly. Recently, second-generation CRISPR/Cas9 activation systems based on nuclease inactive dead (d)Cas9 fused to transcriptional transactivation domains were developed for directing specific guide (g)RNAs to regulatory regions of any gene of interest, to enhance transcription. The application of dCas9 to activate cardiomyocyte transcription in targeted genomic loci in vivo has not been demonstrated so far. Objective: We aimed to develop a mouse model for cardiomyocyte-specific, CRISPR-mediated transcriptional modulation, and to demonstrate its versatility by targeting Mef2d and Klf15 loci (2 well-characterized genes implicated in cardiac hypertrophy and homeostasis) for enhanced transcription. Methods and Results: A mouse model expressing dCas9 with the VPR transcriptional transactivation domains under the control of the Myh (myosin heavy chain) 6 promoter was generated. These mice innocuously expressed dCas9 exclusively in cardiomyocytes. For initial proof-of-concept, we selected Mef2d , which when overexpressed, led to hypertrophy and heart failure, and Klf15 , which is lowly expressed in the neonatal heart. The most effective gRNAs were first identified in fibroblast (C3H/10T1/2) and myoblast (C2C12) cell lines. Using an improved triple gRNA expression system (TRISPR [triple gRNA expression construct]), up to 3 different gRNAs were transduced simultaneously to identify optimal conditions for transcriptional activation. For in vivo delivery of the validated gRNA combinations, we employed systemic administration via adeno-associated virus serotype 9. On gRNA delivery targeting Mef2d expression, we recapitulated the anticipated cardiac hypertrophy phenotype. Using gRNA targeting Klf15 , we could enhance its transcription significantly, although Klf15 is physiologically silenced at that time point. We further confirmed specific and robust dCas9VPR on-target effects. Conclusions: The developed mouse model permits enhancement of gene expression by using endogenous regulatory genomic elements. Proof-of-concept in 2 independent genomic loci suggests versatile applications in controlling transcription in cardiomyocytes of the postnatal heart.

Abstract Aims The multi-C2 domain protein dysferlin localizes to the T-Tubule system of skeletal and heart muscles. In skeletal muscle, dysferlin is known to play a role in membrane repair and in T-tubule biogenesis and maintenance. Dysferlin deficiency manifests as muscular dystrophy of proximal and distal muscles. Cardiomyopathies have been also reported, and some dysferlinopathy mouse models develop cardiac dysfunction under stress. Generally, the role and functional relevance of dysferlin in the heart is not clear. The aim of this study was to analyse the effect of dysferlin deficiency on the transverse–axial tubule system (TATS) structure and on Ca2+ homeostasis in the heart. Methods and results We studied dysferlin localization in rat and mouse cardiomyocytes by immunofluorescence microscopy. In dysferlin-deficient ventricular mouse cardiomyocytes, we analysed the TATS by live staining and assessed Ca2+ handling by patch-clamp experiments and measurement of Ca2+ transients and Ca2+ sparks. We found increasing co-localization of dysferlin with the L-type Ca2+-channel during TATS development and show that dysferlin deficiency leads to pathological loss of transversal and increase in longitudinal elements (axialization). We detected reduced L-type Ca2+-current (ICa,L) in cardiomyocytes from dysferlin-deficient mice and increased frequency of spontaneous sarcoplasmic reticulum Ca2+ release events resulting in pro-arrhythmic contractions. Moreover, cardiomyocytes from dysferlin-deficient mice showed an impaired response to β-adrenergic receptor stimulation. Conclusions Dysferlin is required for TATS biogenesis and maintenance in the heart by controlling the ratio of transversal and axial membrane elements. Absence of dysferlin leads to defects in Ca2+ homeostasis which may contribute to contractile heart dysfunction in dysferlinopathy patients.

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Aims Arrhythmias and sudden cardiac death (SCD) occur commonly in patients with heart failure. We found T-box 5 (TBX5) dysregulated in ventricular myocardium from heart failure patients and thus we hypothesized that TBX5 reduction contributes to arrhythmia development in these patients. To understand the underlying mechanisms, we aimed to reveal the ventricular TBX5-dependent transcriptional network and further test the therapeutic potential of TBX5 level normalization in mice with documented arrhythmias. Methods and Results We used a mouse model of TBX5 conditional deletion in ventricular cardiomyocytes. Ventricular (v) TBX5 loss in mice resulted in mild cardiac dysfunction and arrhythmias and was associated with a high mortality rate (60%) due to SCD. Upon angiotensin stimulation, vTbx5KO mice showed exacerbated cardiac remodelling and dysfunction suggesting a cardioprotective role of TBX5. RNA sequencing of a ventricular specific TBX5KO mouse and TBX5 chromatin immunoprecipitation were used to dissect TBX5 transcriptional network in cardiac ventricular tissue. Overall, we identified 47 transcripts expressed under the control of TBX5, which may have contributed to the fatal arrhythmias in vTbx5KO mice. These included transcripts encoding for proteins implicated in cardiac conduction and contraction (Gja1, Kcnj5, Kcng2, Cacna1g, Chrm2), in cytoskeleton organization (Fstl4, Pdlim4, Emilin2, Cmya5), and cardiac protection upon stress (Fhl2, Gpr22, Fgf16). Interestingly, after TBX5 loss and arrhythmia development in vTbx5KO mice, TBX5 protein level normalization by systemic adeno-associated-virus (AAV) 9 application, re-established TBX5-dependent transcriptome. Consequently, cardiac dysfunction was ameliorated and the propensity of arrhythmia occurrence was reduced. Conclusions This study uncovers a novel cardioprotective role of TBX5 in the adult heart and provides preclinical evidence for the therapeutic value of TBX5 protein normalization in the control of arrhythmia. Translational perspective Cardiovascular disease (CVD) is the number one cause of death worldwide (WHO factsheets 09/2016). Although more than 60% of CVD-related deaths are due to out-of-hospital sudden cardiac death (SCD), we have little insight in the mechanisms underlying SCD pathophysiology. Our data show a link between TBX5 dysregulation and arrhythmia occurrence in patients. To test the therapeutic potential of TBX5, we normalized TBX5 levels in a mouse model with TBX5 dysregulation, which developed arrhythmias and SCD. TBX5 normalization re-established TBX5 target gene expression and more importantly, rescued the arrhythmia phenotype. Altogether, we provide proof-of-concept for the therapeutic potential of TBX5 expression restoration against arrhythmia and SCD.

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Background & Aim: Developmentally conserved pathways, such as the Wnt/β-catenin pathway, are upregulated in adult heart diseases. Here, we aimed to elucidate the role of Wnt regulation via KLF15 for adult heart homeostasis. Results: We identified the transcription factor KLF15, a factor previously identified to contribute to cardiac fibrosis and hypertrophy, as a cardiac specific Wnt/β-catenin inhibitor. Accordingly, a constitutive Klf15 KO mouse model revealed specific cardiac upregulation of the Wnt target genes Tcf7l2 and cMyc (n=6, p<0.001). Serial echocardiography showed reduced systolic function starting at 16 weeks in the Klf15 KO hearts (n=10, p<0.05). Genome-wide sequencing studies of Klf15 KO vs. WT hearts at the postnatal age day P10, week 4 and 20 showed that gene expression differs increasingly at 4 and 20 weeks of age, respectively. This is in line with the increasing KLF15 expression after birth, reaching significant level at P13 (n=3/embryonal, fetal, and neonatal stage; each 5-8 pooled hearts). Accordingly, gene set enrichment analysis revealed activation of the Wnt signaling in Klf15 KO hearts at 4 and 20 weeks but not at P10 (p<0.001). This activation was milder in 20 weeks vs. 4 weeks, which may be explained by a concomitant upregulation of Wnt repressors, as Shisa3 and Dact3 with so far unknown function in the mammalian heart (n=9, p<0,01). Wnt activation was accompanied by upregulation of developmental genes and transcriptional regulators at 4 weeks and followed by upregulation of stress factors such as ANP, BNP, and Ankrd1 at 20 weeks. Moreover, we found the Wnt targets TCF7L2 and AXIN2 significantly upregulated in human ventricular biopsies from dilated (n≥5, p<0.05) and ischemic cardiomyopathy (n≥5, p<0.001) vs. non-failing, further underscoring the relevance of the Wnt pathway for human cardiac cellular homeostasis as well. Conclusion: A Wnt-dependent gene signature may precede the expression of stress factors and functional deterioration due to deletion of its cardiac repressor KLF15. Our data indicate that KLF15 is an important regulator of the Wnt pathway and is relevant for cardiac homeostasis and function in the postnatal heart.