Cardiac circRNAs Arise Mainly From Constitutive Exons Rather Than Alternatively Spliced Exons

Circular RNAs (circRNAs) are a relatively new class of RNA molecules, and knowledge about their biogenesis and function is still in its infancy. It was recently shown that alternative splicing underlies the formation of circular RNAs (circRNA) arising from the Titin (TTN) gene. Since the main mechanism by which circRNAs are formed is still unclear, we hypothesized that alternative splicing, and in particular exon skipping, is a major driver of circRNA production. We performed RNA sequencing on human and mouse hearts, mapped alternative splicing events, and overlaid these with expressed circRNAs at exon-level resolution. In addition, we performed RNA sequencing on hearts of Rbm20 KO mice to address how important Rbm20-mediated alternative splicing is in the production of cardiac circRNAs. In human and mouse hearts, we show that cardiac circRNAs are mostly (~90%) produced from constitutive exons and less (~10%) from alternatively spliced exons. In Rbm20 KO hearts, we identified 38 differentially expressed circRNAs of which 12 were produced from the Ttn gene. Even though Ttn appeared the most prominent target of Rbm20 for circularization, we also detected Rbm20-dependent circRNAs arising from other genes including Fan1, Stk39, Xdh, Bcl2l13, and Sorbs1. Interestingly, only Ttn circRNAs seemed to arise from Rbm20-mediated skipped exons. In conclusion, cardiac circRNAs are mostly derived from constitutive exons, suggesting that these circRNAs are generated at the expense of their linear counterpart and that circRNA production impacts the accumulation of the linear mRNA

The MEF2 transcriptional target DMPK induces loss of sarcomere structure and cardiomyopathy

Aims The pathology of heart failure is characterized by poorly contracting and dilated ventricles. At the cellular level, this is associated with lengthening of individual cardiomyocytes and loss of sarcomeres. While it is known that the transcription factor myocyte enhancer factor-2 (MEF2) is involved in this cardiomyocyte remodelling, the underlying mechanism remains to be elucidated. Here, we aim to mechanistically link MEF2 target genes with loss of sarcomeres during cardiomyocyte remodelling. Methods Neonatal rat cardiomyocytes overexpressing MEF2 elongated and lost their sarcomeric structure. We identified and results myotonic dystrophy protein kinase (DMPK) as direct MEF2 target gene involved in this process. Adenoviral overexpression of DMPK E, the isoform upregulated in heart failure, resulted in severe loss of sarcomeres in vitro, and transgenic mice overexpressing DMPK E displayed disruption of sarcomere structure and cardiomyopathy in vivo. Moreover, we found a decreased expression of sarcomeric genes following DMPK E gain-of-function. These genes are targets of the transcription factor serum response factor (SRF) and we found that DMPK E acts as inhibitor of SRF transcriptional activity. Conclusion Our data indicate that MEF2-induced loss of sarcomeres is mediated by DMPK via a decrease in sarcomeric gene expression by interfering with SRF transcriptional activity. Together, these results demonstrate an unexpected role for DMPK as a direct mediator of adverse cardiomyocyte remodelling and heart failure

The RNA-binding protein Rbm38 is dispensable during pressure overload-induced cardiac remodeling in mice

The importance of tightly controlled alternative pre-mRNA splicing in the heart is emerging. The RNA binding protein Rbm24 has recently been identified as a pivotal cardiac splice factor, which governs sarcomerogenesis in the heart by controlling the expression of alternative protein isoforms. Rbm38, a homolog of Rbm24, has also been implicated in RNA processes such as RNA splicing, RNA stability and RNA translation, but its function in the heart is currently unknown. Here, we investigated the role of Rbm38 in the healthy and diseased adult mouse heart. In contrast to the heart- and skeletal muscle-enriched protein Rbm24, Rbm38 appears to be more broadly expressed. We generated somatic Rbm38 -/- mice and show that global loss of Rbm38 results in hematopoietic defects. Specifically, Rbm38 -/- mice were anemic and displayed enlarged spleens with extramedullary hematopoiesis, as has been shown earlier. The hearts of Rbm38 -/- mice were mildly hypertrophic, but cardiac function was not affected. Furthermore, Rbm38 deficiency did not affect cardiac remodeling (i.e. hypertrophy, LV dilation and fibrosis) or performance (i.e. fractional shortening) after pressure-overload induced by transverse aorta constriction. To further investigate molecular consequences of Rbm38 deficiency, we examined previously identified RNA stability, splicing, and translational targets of Rbm38. We found that stability targets p21 and HuR, splicing targets Mef2d and Fgfr2, and translation target p53 were not altered, suggesting that these Rbm38 targets are tissue-specific or that Rbm38 deficiency may be counteracted by a redundancy mechanism. In this regard, we found a trend towards increased Rbm24 protein expression in Rbm38 -/- hearts. Overall, we conclude that Rbm38 is critical in hematopoiesis, but does not play a critical role in the healthy and diseased heart

New Insights in RBM20 Cardiomyopathy

Purpose of review!#!This review aims to give an update on recent findings related to the cardiac splicing factor RNA-binding motif protein 20 (RBM20) and RBM20 cardiomyopathy, a form of dilated cardiomyopathy caused by mutations in RBM20.!##!Recent findings!#!While most research on RBM20 splicing targets has focused on titin (TTN), multiple studies over the last years have shown that other splicing targets of RBM20 including C

Physiological and unappreciated roles of CaMKII in the heart

In the cardiomyocyte, CaMKII has been identified as a nodal influencer of excitation–contraction and also excitation–transcription coupling. Its activity can be regulated in response to changes in intracellular calcium content as well as after several post-translational modifications. Some of the effects mediated by CaMKII may be considered adaptive, while effects of sustained CaMKII activity may turn into the opposite and are detrimental to cardiac integrity and function. As such, CaMKII has long been noted as a promising target for pharmacological inhibition, but the ubiquitous nature of CaMKII has made it difficult to target CaMKII specifically where it is detrimental. In this review, we provide a brief overview of the physiological and pathophysiological properties of CaMKII signaling, but we focus on the physiological and adaptive functions of CaMKII. Furthermore, special consideration is given to the emerging role of CaMKII as a mediator of inflammatory processes in the heart

RNA Splicing: Regulation and Dysregulation in the Heart

RNA splicing represents a post-transcriptional mechanism to generate multiple functional RNAs or proteins from a single transcript. The evolution of RNA splicing is a prime example of the Darwinian function follows form concept. A mutation that leads to a new mRNA (form) that encodes for a new functional protein (function) is likely to be retained, and this way, the genome has gradually evolved to encode for genes with multiple isoforms, thereby creating an enormously diverse transcriptome. Advances in technologies to characterize RNA populations have led to a better understanding of RNA processing in health and disease. In the heart, alternative splicing is increasingly being recognized as an important layer of post-transcriptional gene regulation. Moreover, the recent identification of several cardiac splice factors, such as RNA-binding motif protein 20 and SF3B1, not only provided important insight into the mechanisms underlying alternative splicing but also revealed how these splicing factors impact functional properties of the heart. Here, we review our current knowledge of alternative splicing in the heart, with a particular focus on the major and minor spliceosome, the factors controlling RNA splicing, and the role of alternative splicing in cardiac development and diseas

Sex differences in distress from infidelity in early adulthood and in later life: A replication and meta-analysis of Shackelford et al. (2014) [Dataset]

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Heterozygous loss of Rbm24 in the adult mouse heart increases sarcomere slack length but does not affect function

RNA-binding proteins are key regulators of post-transcriptional processes such as alternative splicing and mRNA stabilization. Rbm24 acts as a regulator of alternative splicing in heart and skeletal muscle, and is essential for sarcomere assembly. Homozygous inactivation of Rbm24 in mice disrupts cardiac development and results in embryonic lethality around E12.5. In the present study, we generated somatic Rbm24 knockout (KO) mice and investigated the effects of reduced levels of Rbm24 in the adult heart. Due to the embryonic lethality of Rbm24 KO mice, we examined cardiac structure and function in adult Rbm24 heterozygotes (HETs). Rbm24 protein expression was 40% downregulated in HET hearts compared to WT hearts. Force measurements on isolated membrane-permeabilized myocytes showed increased sarcomere slack length and lower myofilament passive stiffness in adult Rbm24 HET compared to wildtype cardiomyocytes. As a result of the differences in sarcomere slack length, the relations between force development and sarcomere length differed between WT and Rbm24 HET hearts. No differences in sarcomere structure and titin isoform composition were observed. Likewise, in vivo cardiac function and myocardial structure was unaltered in Rbm24 HET mice compared to WT, at baseline and upon pressure overload after transverse aortic constriction. In conclusion, we generated a somatic Rbm24 KO model and recapitulated the previously reported embryonic phenotype. In adult Rbm24 HET cardiomyocytes we observed increased sarcomere slack length, but no difference in sarcomere structure and cardiac function

AAV9-mediated Rbm24 overexpression induces fibrosis in the mouse heart

The RNA-binding protein Rbm24 has recently been identified as a pivotal splicing factor in the developing heart. Loss of Rbm24 in mice disrupts cardiac development by governing a large number of muscle-specific splicing events. Since Rbm24 knockout mice are embryonically lethal, the role of Rbm24 in the adult heart remained unexplored. Here, we used adeno-associated viruses (AAV9) to investigate the effect of increased Rbm24 levels in adult mouse heart. Using high-resolution microarrays, we found 893 differentially expressed genes and 1102 differential splicing events in 714 genes in hearts overexpressing Rbm24. We found splicing differences in cardiac genes, such as PDZ and Lim domain 5, Phospholamban, and Titin, but did not find splicing differences in previously identified embryonic splicing targets of Rbm24, such as skNAC, αNAC, and Coro6. Gene ontology enrichment analysis demonstrated increased expression of extracellular matrix (ECM)-related and immune response genes. Moreover, we found increased expression of Tgfβ-signaling genes, suggesting enhanced Tgfβ-signaling in these hearts. Ultimately, this increased activation of cardiac fibroblasts, as evidenced by robust expression of Periostin in the heart, and induced extensive cardiac fibrosis. These results indicate that Rbm24 may function as a regulator of cardiac fibrosis, potentially through the regulation of TgfβR1 and TgfβR2 expression