Contraction of developing avian heart muscle

Fine-tuned KDM1A alternative splicing regulates human cardiomyogenesis through an enzymatic-independent mechanism

The histone demethylase KDM1A is a multi-faceted regulator of vital developmental processes, including mesodermal and cardiac tube formation during gastrulation. However, it is unknown whether the fine-tuning of KDM1A splicing isoforms, already shown to regulate neuronal maturation, is crucial for the specification and maintenance of cell identity during cardiogenesis. Here, we discovered a temporal modulation of ubKDM1A and KDM1A+2a during human and mice fetal cardiac development and evaluated their impact on the regulation of cardiac differentiation. We revealed a severely impaired cardiac differentiation in KDM1A^−/− hESCs that can be rescued by re-expressing ubKDM1A or catalytically impaired ubKDM1A-K661A, but not by KDM1A+2a or KDM1A+2a-K661A. Conversely, KDM1A+2a^−/− hESCs give rise to functional cardiac cells, displaying increased beating amplitude and frequency and enhanced expression of critical cardiogenic markers. Our findings prove the existence of a divergent scaffolding role of KDM1A splice variants, independent of their enzymatic activity, during hESC differentiation into cardiac cells.

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ubKDM1A and KDM1A+2a isoforms are fine-tuned during fetal cardiac development

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Depletion of KDM1A isoforms impairs hESC differentiation into cardiac cells

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KDM1A+2a ablation enhances the expression of key cardiac markers

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KDM1A isoforms exhibit enzymatic-independent divergent roles during cardiogenesis

The Emerging Role of the RBM20 and PTBP1 Ribonucleoproteins in Heart Development and Cardiovascular Diseases

Alternative splicing is a regulatory mechanism essential for cell differentiation and tissue organization. More than 90% of human genes are regulated by alternative splicing events, which participate in cell fate determination. The general mechanisms of splicing events are well known, whereas only recently have deep-sequencing, high throughput analyses and animal models provided novel information on the network of functionally coordinated, tissue-specific, alternatively spliced exons. Heart development and cardiac tissue differentiation require thoroughly regulated splicing events. The ribonucleoprotein RBM20 is a key regulator of the alternative splicing events required for functional and structural heart properties, such as the expression of TTN isoforms. Recently, the polypyrimidine tract-binding protein PTBP1 has been demonstrated to participate with RBM20 in regulating splicing events. In this review, we summarize the updated knowledge relative to RBM20 and PTBP1 structure and molecular function; their role in alternative splicing mechanisms involved in the heart development and function; RBM20 mutations associated with idiopathic dilated cardiovascular disease (DCM); and the consequences of RBM20-altered expression or dysfunction. Furthermore, we discuss the possible application of targeting RBM20 in new approaches in heart therapies.

TNNT2 Missplicing in Skeletal Muscle as a Cardiac Biomarker in Myotonic Dystrophy Type 1 but Not in Myotonic Dystrophy Type 2

Cardiac involvement is one of the most important manifestations of the multisystemic phenotype of patients affected by myotonic dystrophy (DM) and represents the second cause of premature death. Molecular mechanisms responsible for DM cardiac defects are still unclear; however, missplicing of the cardiac isoform of troponin T (TNNT2) and of the cardiac sodium channel (SCN5A) genes might contribute to the reduced myocardial function and conduction abnormalities seen in DM patients. Since, in DM skeletal muscle, the TNNT2 gene shows the same aberrant splicing pattern observed in cardiac muscle, the principal aim of this work was to verify if the TNNT2 aberrant fetal isoform expression could be secondary to myopathic changes or could reflect the DM cardiac phenotype. Analysis of alternative splicing of TNNT2 and of several genes involved in DM pathology has been performed on muscle biopsies from patients affected by DM type 1 (DM1) or type 2 (DM2) with or without cardiac involvement. Our analysis shows that missplicing of muscle-specific genes is higher in DM1 and DM2 than in regenerating control muscles, indicating that these missplicing could be effectively important in DM skeletal muscle pathology. When considering the TNNT2 gene, missplicing appears to be more evident in DM1 than in DM2 muscles since, in DM2, the TNNT2 fetal isoform appears to be less expressed than the adult isoform. This evidence does not seem to be related to less severe muscle histopathological alterations that appear to be similar in DM1 and DM2 muscles. These results seem to indicate that the more severe TNNT2 missplicing observed in DM1 could not be related only to myopathic changes but could reflect the more severe general phenotype compared to DM2, including cardiac problems that appear to be more severe and frequent in DM1 than in DM2 patients. Moreover, TNNT2 missplicing significantly correlates with the QRS cardiac parameter in DM1 but not in DM2 patients, indicating that this splicing event has good potential to function as a biomarker of DM1 severity and it should be considered in pharmacological clinical trials to monitor the possible effects of different therapeutic approaches on skeletal muscle tissues.

Conserved functions of RNA-binding proteins in muscle

Animals require different types of muscle for survival, for example for circulation, motility, reproduction and digestion. Much emphasis in the muscle field has been placed on understanding how transcriptional regulation generates diverse types of muscle during development. Recent work indicates that alternative splicing and RNA regulation are as critical to muscle development, and altered function of RNA-binding proteins causes muscle disease. Although hundreds of genes predicted to bind RNA are expressed in muscles, many fewer have been functionally characterized. We present a cross-species view summarizing what is known about RNA-binding protein function in muscle, from worms and flies to zebrafish, mice and humans. In particular, we focus on alternative splicing regulated by the CELF, MBNL and RBFOX families of proteins. We discuss the systemic nature of diseases associated with loss of RNA-binding proteins in muscle, focusing on mis-regulation of CELF and MBNL in myotonic dystrophy. These examples illustrate the conservation of RNA-binding protein function and the marked utility of genetic model systems in understanding mechanisms of RNA regulation.

Alternative splicing in cardiomyopathy

Alternative splicing is an important mechanism used by the cell to generate greater transcriptomic and proteomic diversity from the genome. In the heart, alternative splicing is increasingly being recognised as an important layer of post-transcriptional gene regulation. Driven by rapidly evolving technologies in next-generation sequencing, alternative splicing has emerged as a crucial process governing complex biological processes during cardiac development and disease. The recent identification of several cardiac splice factors, such as RNA-binding motif protein 20 and 24, 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 factors controlling cardiac alternative splicing and their role in cardiomyopathies and subsequent heart failure.

New Insights Into the Role of RNA-Binding Proteins in the Regulation of Heart Development

The regulation of gene expression during development takes place both at the transcriptional and posttranscriptional levels. RNA-binding proteins (RBPs) regulate pre-mRNA processing, mRNA localization, stability, and translation. Many RBPs are expressed in the heart and have been implicated in heart development, function, or disease. This chapter will review the current knowledge about RBPs in the developing heart, focusing on those that regulate posttranscriptional gene expression. The involvement of RBPs at each stage of heart development will be considered in turn, including the establishment of specific cardiac cell types and formation of the primitive heart tube, cardiac morphogenesis, and postnatal maturation and aging. The contributions of RBPs to cardiac birth defects and heart disease will also be considered in these contexts. Finally, the interplay between RBPs and other regulatory factors in the developing heart, such as transcription factors and miRNAs, will be discussed.

Developmental insights into the pathology of and therapeutic strategies for DM1: Back to the basics

Myotonic Dystrophy type 1 (DM1), the most prevalent adult onset muscular dystrophy, is a trinucleotide repeat expansion disease caused by CTG expansion in the 3'-UTR of DMPK gene. This expansion results in the expression of toxic gain-of-function RNA that forms ribonuclear foci and disrupts normal activities of RNA-binding proteins belonging to the MBNL and CELF families. Changes in alternative splicing, translation, localization, and mRNA stability due to sequestration of MBNL proteins and up-regulation of CELF1 are key to DM1 pathology. However, recent discoveries indicate that pathogenic mechanisms of DM1 involves many other factors as well, including repeat associated translation, activation of PKC-dependent signaling pathway, aberrant polyadenylation, and microRNA deregulation. Expression of the toxic repeat RNA culminates in the developmental remodeling of the transcriptome, which produces fetal isoforms of proteins that are unable to fulfill the physiological requirements of adult tissues. This review will describe advances in the understanding of DM1 pathogenesis as well as current therapeutic developments for DM1.

Alternative splicing and muscular dystrophy

Alternative splicing of pre-mRNAs is a major contributor to proteomic diversity and to the control of gene expression in higher eukaryotic cells. For this reasons, alternative splicing is tightly regulated in different tissues and developmental stages and its disruption can lead to a wide range of human disorders. The aim of this review is to focus on the relevance of alternative splicing for muscle function and muscle disease. We begin by giving a brief overview of alternative splicing, muscle-specific gene expression and muscular dystrophy. Next, to illustrate these concepts we focus on two muscular dystrophy, myotonic muscular dystrophy and facioscapulohumeral muscular dystrophy, both associated to disruption of splicing regulation in muscle.

CELF-mediated alternative splicing is required for cardiac function during early, but not later, postnatal life

During the transition from juvenile to adult life, the heart undergoes programmed remodeling at the levels of transcription and alternative splicing. Members of the CUG-BP and ETR-3-like factor (CELF) family have been implicated in driving developmental transitions in alternative splicing of cardiac transcripts during maturation of the heart. Here, we investigated the timing of the requirement for CELF activity in the postnatal heart using a previously described transgenic mouse model (MHC-CELFDelta). In MHC-CELFDelta mice, nuclear CELF activity has been disrupted specifically in the heart by cardiac-specific expression of a dominant negative CELF protein. Longitudinal analyses of two lines of MHC-CELFDelta mice with differing levels of dominant negative protein expression demonstrate that CELF splicing activity is required for healthy cardiac function during juvenile, but not adult, life. Cardiac function, chamber dilation, and heart size all recover with age in the mild line of MHC-CELFDelta mice without a loss of dominant negative protein expression or change in expression of endogenous CELF proteins or known CELF antagonists. This is the first example of a mouse model with genetically induced cardiomyopathy that spontaneously recovers without intervention. Our results suggest that CELF proteins are key players in the integrated gene expression program involved in postnatal cardiac remodeling and maturation.

Plasticity of cardiac titin/connectin in heart development

Many sarcomeric proteins in the myocardium alter their isoform pattern during perinatal development to adjust to the intensified pump function of the postnatal heart. These changes also involve the giant protein titin/connectin. Here we show by low-percentage polyacrylamide-gel electrophoresis that developmentally regulated switching of cardiac titin/connectin size occurs in the hearts of mouse, rat, pig, and chicken. Mammalian hearts express, well before birth, large foetal (approximately 3.7 MDa) N2BA-titin/connectin isoform but no N2B-isoform (3.0 MDa). During perinatal heart development the 3.7-MDa N2BA-isoform is replaced by a mix of smaller isoforms. At birth a plethora of intermediate-size N2BA-isoforms appears together with the N2B-isoform. In postnatal heart development the larger-size N2BA-isoforms disappear and smaller-size N2BA-isoforms are upregulated, whereas the proportion of N2B-titin/connectin increases to species-specific adult levels. The time courses of isoform switching are faster in small than in large mammals. Titin/connectin isoform switching also takes place in developing chicken hearts, but the largest embryonic isoform found here was less than 3.4 MDa. At hatching, various smaller-size isoforms appeared and within a week the adult expression pattern was established representing a major 3.0-MDa isoform and a minor 3.15-MDa isoform. The ratio between the two adult isoforms differed between the left ventricle and the right atrium. The perinatal changes toward smaller cardiac titin/connectin isoforms in mammals and chicken greatly increase the myofibrillar passive tension of postnatal hearts. Plasticity of titin/connectin at approximately the time of birth thus affects myocardial mechanics but could also be an important factor in developmentally regulated assembly and signalling processes.

Misregulation of alternative splicing causes pathogenesis in myotonic dystrophy

Myotonic dystrophy (DM), the most common form of adult onset muscular dystrophy, affects skeletal muscle, heart, and the central nervous system (CNS). Mortality results primarily from muscle wasting and cardiac arrhythmias. There are two forms of the disease: DM1 and DM2. DM1, which constitutes 98% of cases, is caused by a CTG expansion in the 3' untranslated region (UTR) of the DMPK gene. DM2 is caused by a CCTG expansion in the first intron of the ZNF9 gene. RNA containing CUG- or CCUG-expanded repeats are transcribed but are retained in the nucleus in foci. Disease pathogenesis results primarily from a gain of function of the expanded RNAs, which alter developmentally regulated alternative splicing as well as pathways of muscle differentiation. The toxic RNA has been implicated in sequestration of splicing regulators and transcription factors thereby causing specific symptoms of the disease. Here we review the proposed mechanisms for the toxic effects of the expanded repeats and discuss the molecular mechanisms of splicing misregulation and disease pathogenesis.

Dynamic balance between activation and repression regulates pre-mRNA alternative splicing during heart development

Cardiac troponin T (cTNT) exon 5 splicing is developmentally regulated such that it is included in embryonic but not adult heart. CUG-BP and ETR-3-like factor (CELF) proteins promote exon inclusion, whereas polypyrimidine tract binding protein (PTB) and muscleblind-like (MBNL) proteins repress inclusion. In this study, we addressed what happens to these regulatory proteins during heart development to shift the regulatory balance of cTNT alternative splicing. Using dominant-negative proteins, we found that both CELF and PTB activities are required for appropriate splicing in cardiomyocytes. Two CELF proteins, CUG-BP and ETR-3, are nuclear and cytoplasmic in embryonic heart but are down-regulated in adult heart concomitant with loss of exon inclusion. In contrast, PTB and MBNL1 are expressed throughout heart development. The patterns of cTNT splicing and expression of its regulatory factors are conserved between mouse and chicken. Thus, alternative splicing is determined by a balance between positive and negative regulation, and modulation of expression levels of auxiliary splicing regulators may drive developmental splicing changes. ETR-3 and CUG-BP proteins are also down-regulated in other tissues during development, suggesting that CELF proteins play a broad role in developmental splicing regulation.

Developmental changes in contractility and sarcomeric proteins from the early embryonic to the adult stage in the mouse heart

Developmental changes in force-generating capacity and Ca2+ sensitivity of contraction in murine hearts were correlated with changes in myosin heavy chain (MHC) and troponin (Tn) isoform expression, using Triton-skinned fibres. The maximum Ca2+-activated isometric force normalized to the cross-sectional area (FCSA) increased mainly during embryogenesis and continued to increase at a slower rate until adulthood. During prenatal development, FCSA increased about 5-fold from embryonic day (E)10.5 to E19.5, while the amount of MHC normalized to the amount of total protein remained constant (from E13.5 to E19.5). This suggests that the development of structural organization of the myofilaments during the embryonic and the fetal period may play an important role for the improvement of force generation. There was an overall decrease of 0.5 pCa units in the Ca2+ sensitivity of force generation from E13.5 to the adult, of which the main decrease (0.3 pCa units) occurred within a short time interval, between E19.5 and 7 days after birth (7 days pn). Densitometric analysis of SDS-PAGE and Western blots revealed that the major switches between troponin T (TnT) isoforms occur before E16.5, whereas the transition points of slow skeletal troponin I (ssTnI) to cardiac TnI (cTnI) and of beta-MHC to alpha-MHC both occur around birth, in temporal correlation with the main decrease in Ca2+ sensitivity. To test whether the changes in Ca2+ sensitivity are solely based on Tn, the native Tn complex was replaced in fibres from E19.5 and adult hearts with fast skeletal Tn complex (fsTn) purified from rabbit skeletal muscle. The difference in pre-replacement values of pCa50 (-log([Ca2+] M-1)) required for half-maximum force development) between E19.5 (6.05 +/- 0.01) and adult fibres (5.64 +/- 0.04) was fully abolished after replacement with the exogenous skeletal Tn complex (pCa50 = 6.12 +/- 0.05 for both stages). This suggests that the major developmental changes in Ca2+ sensitivity of skinned murine myocardium originate primarily from the switch of ssTnI to cTnI.

The CELF family of RNA binding proteins is implicated in cell-specific and developmentally regulated alternative splicing

Alternative splicing of cardiac troponin T (cTNT) exon 5 undergoes a developmentally regulated switch such that exon inclusion predominates in embryonic, but not adult, striated muscle. We previously described four muscle-specific splicing enhancers (MSEs) within introns flanking exon 5 in chicken cTNT that are both necessary and sufficient for exon inclusion in embryonic muscle. We also demonstrated that CUG-binding protein (CUG-BP) binds a conserved CUG motif within a human cTNT MSE and positively regulates MSE-dependent exon inclusion. Here we report that CUG-BP is one of a novel family of developmentally regulated RNA binding proteins that includes embryonically lethal abnormal vision-type RNA binding protein 3 (ETR-3). This family, which we call CELF proteins for CUG-BP- and ETR-3-like factors, specifically bound MSE-containing RNAs in vitro and activated MSE-dependent exon inclusion of cTNT minigenes in vivo. The expression of two CELF proteins is highly restricted to brain. CUG-BP, ETR-3, and CELF4 are more broadly expressed, and expression is developmentally regulated in striated muscle and brain. Changes in the level of expression and isoforms of ETR-3 in two different developmental systems correlated with regulated changes in cTNT splicing. A switch from cTNT exon skipping to inclusion tightly correlated with induction of ETR-3 protein expression during differentiation of C2C12 myoblasts. During heart development, the switch in cTNT splicing correlated with a transition in ETR-3 protein isoforms. We propose that ETR-3 is a major regulator of cTNT alternative splicing and that the CELF family plays an important regulatory role in cell-specific alternative splicing during normal development and disease.

Role of cardiac neural crest cells in cardiovascular development

The discovery in the chick embryo that a specific region of the neural crest, termed the cardiac neural crest, is essential for septation of the cardiac outflow tract and for aortic arch artery development has led to the classification of a whole series of human cardiac defects as neural crest-associated. Recently, several mouse genetic models have been effectively employed to yield new insights into the relationship between cardiac neural crest and structural heart development. In all the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, and there are obvious signs of severe cardiac dysfunction. The evidence indicates that poor viability is from impaired cardiac excitation-contraction coupling and contractile function at the myocyte level. The continued study of experimental and genetically defined models with neural crest-associated heart defects will prove useful in identifying the common pathways by which the neural crest contributes to normal heart development.

The effect of 2,3-butanedione 2-monoxime (BDM) on ventricular trabeculae from the avian heart

2,3-butanedione 2-monoxime (BDM, 3-30 mM) decreased twitch force of intact ventricular trabeculae isolated from 19-day embryonic chick hearts in a dose-dependent manner. The responses to BDM were rapid and reversible. In an attempt to determine the cellular basis for the inhibitory effect of BDM, experiments were carried out on skinned muscle fibres and isolated myocytes. In trabeculae skinned with Triton X-100, BDM depressed maximum calcium activated force (Fmax) with an IC50 of 14 mM. At 3 mM BDM, the proportional decrease in twitch force in intact tissue was similar to that of Fmax in skinned tissue. At higher BDM concentrations (10 and 30 mM), however, the proportional decrease in twitch force was greater than that of Fmax. BDM (up to 10 mM) had no effect on the normalized force-pCa relationship. In saponin-skinned preparations, BDM (3 and 30 mM) released calcium from the fully loaded sarcoplasmic reticulum to a slightly greater extent in the absence of calcium (pCa 8.5) than in the presence of a fixed level of free calcium (pCa 5.5). Whole cell patch clamping of freshly isolated chick myocytes demonstrated that BDM caused a dose-dependent decrease in the T- and L-type calcium current. Therefore, at low BDM concentrations (3 mM), the decrease in twitch force can be ascribed predominantly to depression of the contractile apparatus while, at higher concentrations of BDM, there is an additional inhibitory effect of BDM on excitation-contraction coupling.