PKC inhibition ameliorates the cardiac phenotype in a mouse model of myotonic dystrophy type 1

Therapeutic potential of oleic acid supplementation in myotonic dystrophy muscle cell models

BACKGROUND

We recently reported that upregulation of Musashi 2 (MSI2) protein in the rare neuromuscular disease myotonic dystrophy type 1 contributes to the hyperactivation of the muscle catabolic processes autophagy and UPS through a reduction in miR-7 levels. Because oleic acid (OA) is a known allosteric regulator of MSI2 activity in the biogenesis of miR-7, here we sought to evaluate endogenous levels of this fatty acid and its therapeutic potential in rescuing cell differentiation phenotypes in vitro. In this work, four muscle cell lines derived from DM1 patients were treated with OA for 24 h, and autophagy and muscle differentiation parameters were analyzed.

RESULTS

We demonstrate a reduction of OA levels in different cell models of the disease. OA supplementation rescued disease-related phenotypes such as fusion index, myotube diameter, and repressed autophagy. This involved inhibiting MSI2 regulation of direct molecular target miR-7 since OA isoschizomer, elaidic acid (EA) could not cause the same rescues. Reduction of OA levels seems to stem from impaired biogenesis since levels of the enzyme stearoyl-CoA desaturase 1 (SCD1), responsible for converting stearic acid to oleic acid, are decreased in DM1 and correlate with OA amounts.

CONCLUSIONS

For the first time in DM1, we describe a fatty acid metabolism impairment that originated, at least in part, from a decrease in SCD1. Because OA allosterically inhibits MSI2 binding to molecular targets, reduced OA levels synergize with the overexpression of MSI2 and contribute to the MSI2 > miR-7 > autophagy axis that we proposed to explain the muscle atrophy phenotype.

Cardiac involvement in patient-specific induced pluripotent stem cells of myotonic dystrophy type 1: unveiling the impact of voltage-gated sodium channels

Myotonic dystrophy type 1 (DM1) is a genetic disorder that causes muscle weakness and myotonia. In DM1 patients, cardiac electrical manifestations include conduction defects and atrial fibrillation. DM1 results in the expansion of a CTG transcribed into CUG-containing transcripts that accumulate in the nucleus as RNA foci and alter the activity of several splicing regulators. The underlying pathological mechanism involves two key RNA-binding proteins (MBNL and CELF) with expanded CUG repeats that sequester MBNL and alter the activity of CELF resulting in spliceopathy and abnormal electrical activity. In the present study, we identified two DM1 patients with heart conduction abnormalities and characterized their hiPSC lines. Two differentiation protocols were used to investigate both the ventricular and the atrial electrophysiological aspects of DM1 and unveil the impact of the mutation on voltage-gated ion channels, electrical activity, and calcium homeostasis in DM1 cardiomyocytes derived from hiPSCs. Our analysis revealed the presence of molecular hallmarks of DM1, including the accumulation of RNA foci and sequestration of MBNL1 in DM1 hiPSC-CMs. We also observed mis-splicing of SCN5A and haploinsufficiency of DMPK. Furthermore, we conducted separate characterizations of atrial and ventricular electrical activity, conduction properties, and calcium homeostasis. Both DM1 cell lines exhibited reduced density of sodium and calcium currents, prolonged action potential duration, slower conduction velocity, and impaired calcium transient propagation in both ventricular and atrial cardiomyocytes. Notably, arrhythmogenic events were recorded, including both ventricular and atrial arrhythmias were observed in the two DM1 cell lines. These findings enhance our comprehension of the molecular mechanisms underlying DM1 and provide valuable insights into the pathophysiology of ventricular and atrial involvement.

Protein Phosphorylation Alterations in Myotonic Dystrophy Type 1: A Systematic Review

Among the most common muscular dystrophies in adults is Myotonic Dystrophy type 1 (DM1), an autosomal dominant disorder characterized by myotonia, muscle wasting and weakness, and multisystemic dysfunctions. This disorder is caused by an abnormal expansion of the CTG triplet at the DMPK gene that, when transcribed to expanded mRNA, can lead to RNA toxic gain of function, alternative splicing impairments, and dysfunction of different signaling pathways, many regulated by protein phosphorylation. In order to deeply characterize the protein phosphorylation alterations in DM1, a systematic review was conducted through PubMed and Web of Science databases. From a total of 962 articles screened, 41 were included for qualitative analysis, where we retrieved information about total and phosphorylated levels of protein kinases, protein phosphatases, and phosphoproteins in DM1 human samples and animal and cell models. Twenty-nine kinases, 3 phosphatases, and 17 phosphoproteins were reported altered in DM1. Signaling pathways that regulate cell functions such as glucose metabolism, cell cycle, myogenesis, and apoptosis were impaired, as seen by significant alterations to pathways such as AKT/mTOR, MEK/ERK, PKC/CUGBP1, AMPK, and others in DM1 samples. This explains the complexity of DM1 and its different manifestations and symptoms, such as increased insulin resistance and cancer risk. Further studies can be done to complement and explore in detail specific pathways and how their regulation is altered in DM1, to find what key phosphorylation alterations are responsible for these manifestations, and ultimately to find therapeutic targets for future treatments.

Molecular Therapies for Myotonic Dystrophy Type 1: From Small Drugs to Gene Editing

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy affecting many different body tissues, predominantly skeletal and cardiac muscles and the central nervous system. The expansion of CTG repeats in the DM1 protein-kinase (DMPK) gene is the genetic cause of the disease. The pathogenetic mechanisms are mainly mediated by the production of a toxic expanded CUG transcript from the DMPK gene. With the availability of new knowledge, disease models, and technical tools, much progress has been made in the discovery of altered pathways and in the potential of therapeutic intervention, making the path to the clinic a closer reality. In this review, we describe and discuss the molecular therapeutic strategies for DM1, which are designed to directly target the CTG genomic tract, the expanded CUG transcript or downstream signaling molecules.

Myotonic Dystrophies: A Genetic Overview

Myotonic dystrophies (DM) are the most common muscular dystrophies in adults, which can affect other non-skeletal muscle organs such as the heart, brain and gastrointestinal system. There are two genetically distinct types of myotonic dystrophy: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2), both dominantly inherited with significant overlap in clinical manifestations. DM1 results from CTG repeat expansions in the 3'-untranslated region (3'UTR) of the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19, while DM2 is caused by CCTG repeat expansions in intron 1 of the CNBP (cellular nucleic acid-binding protein) gene on chromosome 3. Recent advances in genetics and molecular biology, especially in the field of RNA biology, have allowed better understanding of the potential pathomechanisms involved in DM. In this review article, core clinical features and genetics of DM are presented followed by a discussion on the current postulated pathomechanisms and therapeutic approaches used in DM, including the ones currently in human clinical trial phase.

The Role of Junctophilin Proteins in Cellular Function

Junctophilins (JPHs) comprise a family of structural proteins that connect the plasma membrane to intracellular organelles such as the endo/sarcoplasmic reticulum. Tethering of these membrane structures results in the formation of highly organized subcellular junctions that play important signaling roles in all excitable cell types. There are four JPH isoforms, expressed primarily in muscle and neuronal cell types. Each JPH protein consists of 6 'membrane occupation and recognition nexus' (MORN) motifs, a joining region connecting these to another set of 2 MORN motifs, a putative alpha-helical region, a divergent region exhibiting low homology between JPH isoforms, and a carboxy-terminal transmembrane region anchoring into the ER/SR membrane. JPH isoforms play essential roles in developing and maintaining subcellular membrane junctions. Conversely, inherited mutations in JPH2 cause hypertrophic or dilated cardiomyopathy, while trinucleotide expansions in the JPH3 gene cause Huntington Disease-Like 2. Loss of JPH1 protein levels can cause skeletal myopathy, while loss of cardiac JPH2 levels causes heart failure and atrial fibrillation, among other disease. This review will provide a comprehensive overview of the JPH gene family, phylogeny, and evolutionary analysis of JPH genes and other MORN domain proteins. JPH biogenesis, membrane tethering, and binding partners will be discussed, as well as functional roles of JPH isoforms in excitable cells. Finally, potential roles of JPH isoform deficits in human disease pathogenesis will be reviewed.

Brain Pathogenesis and Potential Therapeutic Strategies in Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy that affects multiple systems including the muscle and heart. The mutant CTG expansion at the 3'-UTR of the DMPK gene causes the expression of toxic RNA that aggregate as nuclear foci. The foci then interfere with RNA-binding proteins, affecting hundreds of mis-spliced effector genes, leading to aberrant alternative splicing and loss of effector gene product functions, ultimately resulting in systemic disorders. In recent years, increasing clinical, imaging, and pathological evidence have indicated that DM1, though to a lesser extent, could also be recognized as true brain diseases, with more and more researchers dedicating to develop novel therapeutic tools dealing with it. In this review, we summarize the current advances in the pathogenesis and pathology of central nervous system (CNS) deficits in DM1, intervention measures currently being investigated are also highlighted, aiming to promote novel and cutting-edge therapeutic investigations.

CELF1 promotes vascular endothelial growth factor degradation resulting in impaired microvasculature in heart failure

Vascular rarefaction due to impaired angiogenesis is associated with contractile dysfunction and the transition from compensation to decompensation and heart failure. The regulatory mechanism controlling vascular rarefaction during the transition remains elusive. Increased expression of a nuclear RNA-binding protein CUGBP Elav-like family member 1 (CELF1) in the adult heart is associated with the transition from compensated hypertrophy to decompensated heart failure. Elevated CELF1 level resulted in degradation of the major cardiac gap junction protein, connexin 43, in dilated cardiomyopathy (DCM), the most common cause of heart failure. In the present study, we investigated the role of increased CELF1 expression in causing vascular rarefaction in DCM. CELF1 overexpression (CELF1-OE) in cardiomyocytes resulted in reduced capillary density. CELF1-OE mice administered hypoxyprobe showed immunoreactivity and increased mRNA levels of HIF1α, Glut-1, and Pdk-1, which suggested the association of a reduced capillary density-induced hypoxic condition with CELF1 overexpression. Vegfa mRNA level was downregulated in mouse hearts exhibiting DCM, including CELF1-OE and infarcted hearts. Vegfa mRNA level was also downregulated to a similar extent in cardiomyocytes isolated from infarcted hearts by Langendorff preparation, which suggested cardiomyocyte-derived Vegfa expression mediated by CELF1. Cardiomyocyte-specific depletion of CELF1 preserved the capillary density and Vegfa mRNA level in infarcted mouse hearts. Also, CELF1 bound to Vegfa mRNA and regulated Vegfa mRNA stability via the 3' untranslated region. These results suggest that elevated CELF1 level has dual effects on impairing the functions of cardiomyocytes and microvasculature in DCM.

The hallmarks of myotonic dystrophy type 1 muscle dysfunction

Myotonic dystrophy type 1 (DM1) is the most prevalent form of muscular dystrophy in adults and yet there are currently no treatment options. Although this disease causes multisystemic symptoms, it is mainly characterised by myopathy or diseased muscles, which includes muscle weakness, atrophy, and myotonia, severely affecting the lives of patients worldwide. On a molecular level, DM1 is caused by an expansion of CTG repeats in the 3' untranslated region (3'UTR) of the DM1 Protein Kinase (DMPK) gene which become pathogenic when transcribed into RNA forming ribonuclear foci comprised of auto complementary CUG hairpin structures that can bind proteins. This leads to the sequestration of the muscleblind-like (MBNL) family of proteins, depleting them, and the abnormal stabilisation of CUGBP Elav-like family member 1 (CELF1), enhancing it. Traditionally, DM1 research has focused on this RNA toxicity and how it alters MBNL and CELF1 functions as key splicing regulators. However, other proteins are affected by the toxic DMPK RNA and there is strong evidence that supports various signalling cascades playing an important role in DM1 pathogenesis. Specifically, the impairment of protein kinase B (AKT) signalling in DM1 increases autophagy, apoptosis, and ubiquitin-proteasome activity, which may also be affected in DM1 by AMP-activated protein kinase (AMPK) downregulation. AKT also regulates CELF1 directly, by affecting its subcellular localisation, and indirectly as it inhibits glycogen synthase kinase 3 beta (GSK3β), which stabilises the repressive form of CELF1 in DM1. Another kinase that contributes to CELF1 mis-regulation, in this case by hyperphosphorylation, is protein kinase C (PKC). Additionally, it has been demonstrated that fibroblast growth factor-inducible 14 (Fn14) is induced in DM1 and is associated with downstream signalling through the nuclear factor κB (NFκB) pathways, associating inflammation with this disease. Furthermore, MBNL1 and CELF1 play a role in cytoplasmic processes involved in DM1 myopathy, altering proteostasis and sarcomere structure. Finally, there are many other elements that could contribute to the muscular phenotype in DM1 such as alterations to satellite cells, non-coding RNA metabolism, calcium dysregulation, and repeat-associated non-ATG (RAN) translation. This review aims to organise the currently dispersed knowledge on the different pathways affected in DM1 and discusses the unexplored connections that could potentially help in providing new therapeutic targets in DM1 research.

miR-322/-503 rescues myoblast defects in myotonic dystrophy type 1 cell model by targeting CUG repeats

Myotonic dystrophy type 1 (DM1) is the most common type of adult muscular dystrophy caused by the expanded triple-nucleotides (CUG) repeats. Myoblast in DM1 displayed many defects, including defective myoblast differentiation, ribonuclear foci, and aberrant alternative splicing. Despite many were revealed to function in DM1, microRNAs that regulated DM1 via directly targeting the expanded CUG repeats were rarely reported. Here we discovered that miR-322/-503 rescued myoblast defects in DM1 cell model by targeting the expanded CUG repeats. First, we studied the function of miR-322/-503 in normal C2C12 myoblast cells. Downregulation of miR-322/-503 significantly hindered the myoblast differentiation, while miR-322/-503 overexpression promoted the process. Next, we examined the role of miR-322/-503 in the DM1 C2C12 cell model. miR-322/-503 was downregulated in the differentiation of DM1 C2C12 cells. When we introduced ectopic miR-322/-503 expression into DM1 C2C12 cells, myoblast defects were almost fully rescued, marked by significant improvements of myoblast differentiation and repressions of ribonuclear foci formation and aberrant alternative splicing. Then we investigated the downstream mechanism of miR-322/-503 in DM1. Agreeing with our previous work, Celf1 was proven to be miR-322/-503′s target. Celf1 knockdown partially reproduced miR-322/-503′s function in rescuing DM1 C2C12 differentiation but was unable to repress ribonuclear foci, suggesting other targets of miR-322/-503 existed in the DM1 C2C12 cells. As the seed regions of miR-322 and miR-503 were complementary to the CUG repeats, we hypothesized that the CUG repeats were the target of miR-322/-503. Through expression tests, reporter assays, and colocalization staining, miR-322/-503 was proved to directly and specifically target the expanded CUG repeats in the DM1 cell model rather than the shorter ones in normal cells. Those results implied a potential therapeutic function of miR-322/-503 on DM1, which needed further investigations in the future.

Systemic therapy in an RNA toxicity mouse model with an antisense oligonucleotide therapy targeting a non-CUG sequence within the DMPK 3'UTR RNA

Myotonic dystrophy type 1 (DM1), the most common adult muscular dystrophy, is an autosomal dominant disorder caused by an expansion of a (CTG)n tract within the 3' untranslated region (3'UTR) of the dystrophia myotonica protein kinase (DMPK) gene. Mutant DMPK mRNAs are toxic, present in nuclear RNA foci and correlated with a plethora of RNA splicing defects. Cardinal features of DM1 are myotonia and cardiac conduction abnormalities. Using transgenic mice, we have demonstrated that expression of the mutant DMPK 3'UTR is sufficient to elicit these features of DM1. Here, using these mice, we present a study of systemic treatment with an antisense oligonucleotide (ASO) (ISIS 486178) targeted to a non-CUG sequence within the 3'UTR of DMPK. RNA foci and DMPK 3'UTR mRNA levels were reduced in both the heart and skeletal muscles. This correlated with improvements in several splicing defects in skeletal and cardiac muscles. The treatment reduced myotonia and this correlated with increased Clcn1 expression. Furthermore, functional testing showed improvements in treadmill running. Of note, we demonstrate that the ASO treatment reversed the cardiac conduction abnormalities, and this correlated with restoration of Gja5 (connexin 40) expression in the heart. This is the first time that an ASO targeting a non-CUG sequence within the DMPK 3'UTR has demonstrated benefit on the key DM1 phenotypes of myotonia and cardiac conduction defects. Our data also shows for the first time that ASOs may be a viable option for treating cardiac pathology in DM1.

Aberrant Expression of a Non-muscle RBFOX2 Isoform Triggers Cardiac Conduction Defects in Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1) is a multisystemic genetic disorder caused by the CTG repeat expansion in the 3'-untranslated region of DMPK gene. Heart dysfunctions occur in ∼80% of DM1 patients and are the second leading cause of DM1-related deaths. Herein, we report that upregulation of a non-muscle splice isoform of RNA-binding protein RBFOX2 in DM1 heart tissue-due to altered splicing factor and microRNA activities-induces cardiac conduction defects in DM1 individuals. Mice engineered to express the non-muscle RBFOX240 isoform in heart via tetracycline-inducible transgenesis, or CRISPR/Cas9-mediated genome editing, reproduced DM1-related cardiac conduction delay and spontaneous episodes of arrhythmia. Further, by integrating RNA binding with cardiac transcriptome datasets from DM1 patients and mice expressing the non-muscle RBFOX2 isoform, we identified RBFOX240-driven splicing defects in voltage-gated sodium and potassium channels, which alter their electrophysiological properties. Thus, our results uncover a trans-dominant role for an aberrantly expressed RBFOX240 isoform in DM1 cardiac pathogenesis.

Correction of RNA-Binding Protein CUGBP1 and GSK3β Signaling as Therapeutic Approach for Congenital and Adult Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is a complex genetic disease affecting many tissues. DM1 is caused by an expansion of CTG repeats in the 3′-UTR of the DMPK gene. The mechanistic studies of DM1 suggested that DMPK mRNA, containing expanded CUG repeats, is a major therapeutic target in DM1. Therefore, the removal of the toxic RNA became a primary focus of the therapeutic development in DM1 during the last decade. However, a cure for this devastating disease has not been found. Whereas the degradation of toxic RNA remains a preferential approach for the reduction of DM1 pathology, other approaches targeting early toxic events downstream of the mutant RNA could be also considered. In this review, we discuss the beneficial role of the restoring of the RNA-binding protein, CUGBP1/CELF1, in the correction of DM1 pathology. It has been recently found that the normalization of CUGBP1 activity with the inhibitors of GSK3 has a positive effect on the reduction of skeletal muscle and CNS pathologies in DM1 mouse models. Surprisingly, the inhibitor of GSK3, tideglusib also reduced the toxic CUG-containing RNA. Thus, the development of the therapeutics, based on the correction of the GSK3β-CUGBP1 pathway, is a promising option for this complex disease.

Straightjacket/α2δ3 deregulation is associated with cardiac conduction defects in myotonic dystrophy type 1

Cardiac conduction defects decrease life expectancy in myotonic dystrophy type 1 (DM1), a CTG repeat disorder involving misbalance between two RNA binding factors, MBNL1 and CELF1. However, how DM1 condition translates into conduction disorders remains poorly understood. Here we simulated MBNL1 and CELF1 misbalance in the Drosophila heart and performed TU-tagging-based RNAseq of cardiac cells. We detected deregulations of several genes controlling cellular calcium levels, including increased expression of straightjacket/α2δ3, which encodes a regulatory subunit of a voltage-gated calcium channel. Straightjacket overexpression in the fly heart leads to asynchronous heartbeat, a hallmark of abnormal conduction, whereas cardiac straightjacket knockdown improves these symptoms in DM1 fly models. We also show that ventricular α2δ3 expression is low in healthy mice and humans, but significantly elevated in ventricular muscles from DM1 patients with conduction defects. These findings suggest that reducing ventricular straightjacket/α2δ3 levels could offer a strategy to prevent conduction defects in DM1.

Mitigating RNA Toxicity in Myotonic Dystrophy using Small Molecules

This review, one in a series on myotonic dystrophy (DM), is focused on the development and potential use of small molecules as therapeutics for DM. The complex mechanisms and pathogenesis of DM are covered in the associated reviews. Here, we examine the various small molecule approaches taken to target the DNA, RNA, and proteins that contribute to disease onset and progression in myotonic dystrophy type 1 (DM1) and 2 (DM2).

Deprivation of Muscleblind-Like Proteins Causes Deficits in Cortical Neuron Distribution and Morphological Changes in Dendritic Spines and Postsynaptic Densities

Myotonic dystrophy (Dystrophia Myotonica; DM) is the most common adult-onset muscular dystrophy and its brain symptoms seriously affect patients’ quality of life. It is caused by extended (CTG)_n expansions at 3′-UTR of DMPK gene (DM type 1, DM1) or (CCTG)_n repeats in the intron 1 of CNBP gene (DM type 2, DM2) and the sequestration of Muscleblind-like (MBNL) family proteins by transcribed (CUG)_n RNA hairpin is the main pathogenic mechanism for DM. The MBNL proteins are splicing factors regulating posttranscriptional RNA during development. Previously, Mbnl knockout (KO) mouse lines showed molecular and phenotypic evidence that recapitulate DM brains, however, detailed morphological study has not yet been accomplished. In our studies, control (Mbnl1^+/+; Mbnl2^cond/cond; Nestin-Cre^−/−), Mbnl2 conditional KO (2KO, Mbnl1^+/+; Mbnl2^cond/cond; Nestin-Cre^+/−) and Mbnl1/2 double KO (DKO, Mbnl1^ΔE3/ΔE3; Mbnl2^cond/cond; Nestin-Cre^+/−) mice were generated by crossing three individual lines. Immunohistochemistry for evaluating density and distribution of cortical neurons; Golgi staining for depicting the dendrites/dendritic spines; and electron microscopy for analyzing postsynaptic ultrastructure were performed. We found distributional defects in cortical neurons, reduction in dendritic complexity, immature dendritic spines and alterations of postsynaptic densities (PSDs) in the mutants. In conclusion, loss of function of Mbnl1/2 caused fundamental defects affecting neuronal distribution, dendritic morphology and postsynaptic architectures that are reminiscent of predominantly immature and fetal phenotypes in DM patients.

Global Increase in Circular RNA Levels in Myotonic Dystrophy

Splicing aberrations induced as a consequence of the sequestration of muscleblind-like splicing factors on the dystrophia myotonica protein kinase transcript, which contains expanded CUG repeats, present a major pathomechanism of myotonic dystrophy type 1 (DM1). As muscleblind-like factors may also be important factors involved in the biogenesis of circular RNAs (circRNAs), we hypothesized that the level of circRNAs would be decreased in DM1. To test this hypothesis, we selected 20 well-validated circRNAs and analyzed their levels in several experimental systems (e.g., cell lines, DM muscle tissues, and a mouse model of DM1) using droplet digital PCR assays. We also explored the global level of circRNAs using two RNA-Seq datasets of DM1 muscle samples. Contrary to our original hypothesis, our results consistently showed a global increase in circRNA levels in DM1, and we identified numerous circRNAs that were increased in DM1. We also identified many genes (including muscle-specific genes) giving rise to numerous (>10) circRNAs. Thus, this study is the first to show an increase in global circRNA levels in DM1. We also provided preliminary results showing the association of circRNA level with muscle weakness and alternative splicing changes that are biomarkers of DM1 severity.

Detection of expanded RNA repeats using thermostable group II intron reverse transcriptase

Cellular accumulation of repetitive RNA occurs in several dominantly-inherited genetic disorders. Expanded CUG, CCUG or GGGGCC repeats are expressed in myotonic dystrophy type 1 (DM1), myotonic dystrophy type 2 (DM2), or familial amyotrophic lateral sclerosis, respectively. Expanded repeat RNAs (ER-RNAs) exert a toxic gain-of-function and are prime therapeutic targets in these diseases. However, efforts to quantify ER-RNA levels or monitor knockdown are confounded by stable structure and heterogeneity of the ER-RNA tract and background signal from non-expanded repeats. Here, we used a thermostable group II intron reverse transcriptase (TGIRT-III) to convert ER-RNA to cDNA, followed by quantification on slot blots. We found that TGIRT-III was capable of reverse transcription (RTn) on enzymatically synthesized ER-RNAs. By using conditions that limit cDNA synthesis from off-target sequences, we observed hybridization signals on cDNA slot blots from DM1 and DM2 muscle samples but not from healthy controls. In transgenic mouse models of DM1 the cDNA slot blots accurately reflected the differences of ER-RNA expression across different transgenic lines, and showed therapeutic reductions in skeletal and cardiac muscle, accompanied by improvements of the DM1-associated splicing defects. TGIRT-III was also active on CCCCGG- and GGGGCC-repeats, suggesting that ER-RNA analysis is feasible for several repeat expansion disorders.

Cardiovascular manifestations of myotonic dystrophy

Patients with myotonic dystrophy, the most common neuromuscular dystrophy in adults, have a high prevalence of arrhythmic complications with increased cardiovascular mortality and high risk for sudden death. Sudden death prevention is central and relies on annual follow-up and prophylactic permanent pacing in patients with conduction defects on electrocardiogram and/or infrahisian blocks on electrophysiological study. Implantable cardiac defibrillator therapy may be indicated in patients with ventricular tachyarrhythmia.

PKC and PKN in heart disease

The protein kinase C (PKC) and closely related protein kinase N (PKN) families of serine/threonine protein kinases play crucial cellular roles. Both kinases belong to the AGC subfamily of protein kinases that also include the cAMP dependent protein kinase (PKA), protein kinase B (PKB/AKT), protein kinase G (PKG) and the ribosomal protein S6 kinase (S6K). Involvement of PKC family members in heart disease has been well documented over the years, as their activity and levels are mis-regulated in several pathological heart conditions, such as ischemia, diabetic cardiomyopathy, as well as hypertrophic or dilated cardiomyopathy. This review focuses on the regulation of PKCs and PKNs in different pathological heart conditions and on the influences that PKC/PKN activation has on several physiological processes. In addition, we discuss mechanisms by which PKCs and the closely related PKNs are activated and turned-off in hearts, how they regulate cardiac specific downstream targets and pathways, and how their inhibition by small molecules is explored as new therapeutic target to treat cardiomyopathies and heart failure.

Of Mice and Men: Advances in the Understanding of Neuromuscular Aspects of Myotonic Dystrophy

Intensive effort has been directed toward the modeling of myotonic dystrophy (DM) in mice, in order to reproduce human disease and to provide useful tools to investigate molecular and cellular pathogenesis and test efficient therapies. Mouse models have contributed to dissect the multifaceted impact of the DM mutation in various tissues, cell types and in a pleiotropy of pathways, through the expression of toxic RNA transcripts. Changes in alternative splicing, transcription, translation, intracellular RNA localization, polyadenylation, miRNA metabolism and phosphorylation of disease intermediates have been described in different tissues. Some of these events have been directly associated with specific disease symptoms in the skeletal muscle and heart of mice, offering the molecular explanation for individual disease phenotypes. In the central nervous system (CNS), however, the situation is more complex. We still do not know how the molecular abnormalities described translate into CNS dysfunction, nor do we know if the correction of individual molecular events will provide significant therapeutic benefits. The variability in model design and phenotypes described so far requires a thorough and critical analysis. In this review we discuss the recent contributions of mouse models to the understanding of neuromuscular aspects of disease, therapy development, and we provide a reflective assessment of our current limitations and pressing questions that remain unanswered.

Small Molecules Which Improve Pathogenesis of Myotonic Dystrophy Type 1

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy in adults for which there is currently no treatment. The pathogenesis of this autosomal dominant disorder is associated with the expansion of CTG repeats in the 3'-UTR of the DMPK gene. DMPK transcripts with expanded CUG repeats (CUGexpDMPK) are retained in the nucleus forming multiple discrete foci, and their presence triggers a cascade of toxic events. Thus far, most research emphasis has been on interactions of CUGexpDMPK with the muscleblind-like (MBNL) family of splicing factors. These proteins are sequestered by the expanded CUG repeats of DMPK RNA leading to their functional depletion. As a consequence, abnormalities in many pathways of RNA metabolism, including alternative splicing, are detected in DM1. To date, in vitro and in vivo efforts to develop therapeutic strategies for DM1 have mostly been focused on targeting CUGexpDMPK via reducing their expression and/or preventing interactions with MBNL1. Antisense oligonucleotides targeted to the CUG repeats in the DMPK transcripts are of particular interest due to their potential capacity to discriminate between mutant and normal transcripts. However, a growing number of reports describe alternative strategies using small molecule chemicals acting independently of a direct interaction with CUGexpDMPK. In this review, we summarize current knowledge about these chemicals and we describe the beneficial effects they caused in different DM1 experimental models. We also present potential mechanisms of action of these compounds and pathways they affect which could be considered for future therapeutic interventions in DM1.

Daunorubicin reduces MBNL1 sequestration caused by CUG-repeat expansion and rescues cardiac dysfunctions in a Drosophila model of myotonic dystrophy

Myotonic dystrophy (DM) is a dominantly inherited neuromuscular disorder caused by expression of mutant myotonin-protein kinase (DMPK) transcripts containing expanded CUG repeats. Pathogenic DMPK RNA sequesters the muscleblind-like (MBNL) proteins, causing alterations in metabolism of various RNAs. Cardiac dysfunction represents the second most common cause of death in DM type 1 (DM1) patients. However, the contribution of MBNL sequestration in DM1 cardiac dysfunction is unclear. We overexpressed Muscleblind (Mbl), the DrosophilaMBNL orthologue, in cardiomyocytes of DM1 model flies and observed a rescue of heart dysfunctions, which are characteristic of these model flies and resemble cardiac defects observed in patients. We also identified a drug – daunorubicin hydrochloride – that directly binds to CUG repeats and alleviates Mbl sequestration in Drosophila DM1 cardiomyocytes, resulting in mis-splicing rescue and cardiac function recovery. These results demonstrate the relevance of Mbl sequestration caused by expanded-CUG-repeat RNA in cardiac dysfunctions in DM1, and highlight the potential of strategies aimed at inhibiting this protein-RNA interaction to recover normal cardiac function.

Summary: MBNL protein sequestration by expanded CUG RNA contributes towards cardiac dysfunction in a myotonic dystrophy Drosophila model. Here, the authors identify the anticancer drug daunorubicin as a candidate therapeutic for the disease.

Myotonic Dystrophy and Developmental Regulation of RNA Processing

Myotonic dystrophy (DM) is a multisystemic disorder caused by microsatellite expansion mutations in two unrelated genes leading to similar, yet distinct, diseases. DM disease presentation is highly variable and distinguished by differences in age-of-onset and symptom severity. In the most severe form, DM presents with congenital onset and profound developmental defects. At the molecular level, DM pathogenesis is characterized by a toxic RNA gain-of-function mechanism that involves the transcription of noncoding microsatellite expansions. These mutant RNAs disrupt key cellular pathways, including RNA processing, localization, and translation. In DM, these toxic RNA effects are predominantly mediated through the modulation of the muscleblind-like and CUGBP and ETR-3-like factor families of RNA binding proteins (RBPs). Dysfunction of these RBPs results in widespread RNA processing defects culminating in the expression of developmentally inappropriate protein isoforms in adult tissues. The tissue that is the focus of this review, skeletal muscle, is particularly sensitive to mutant RNA-responsive perturbations, as patients display a variety of developmental, structural, and functional defects in muscle. Here, we provide a comprehensive overview of DM1 and DM2 clinical presentation and pathology as well as the underlying cellular and molecular defects associated with DM disease onset and progression. Additionally, fundamental aspects of skeletal muscle development altered in DM are highlighted together with ongoing and potential therapeutic avenues to treat this muscular dystrophy. © 2018 American Physiological Society. Compr Physiol 8:509-553, 2018.

CELF1 Mediates Connexin 43 mRNA Degradation in Dilated Cardiomyopathy

Rationale:

Downregulation of Cx43 (connexin 43), the major cardiac gap junction protein, is often associated with arrhythmia, dilated cardiomyopathy (DCM), and heart failure. However, the cause of the reduced expression remains elusive. Reinduction of a nuclear RNA-binding protein CELF1 (CUGBP Elav-like family member 1) in the adult heart has been implicated in the cardiac pathogenesis of myotonic dystrophy type 1. However, how elevated CELF1 level leads to cardiac dysfunction, such as conduction defect, DCM, and heart failure, remains unclear.

Objective:

We investigated the mechanism of CELF1-mediated Cx43 mRNA degradation and determined whether elevated CELF1 expression is also a shared feature of the DCM heart.

Methods and Results:

RNA immunoprecipitation revealed the involvement of CELF1-regulated genes, including Cx43, in controlling contractility and conduction. CELF1 mediated Cx43 mRNA degradation by binding the UG-rich element in the 3′ untranslated region of Cx43. Mutation of the nuclear localization signal in CELF1 abolished the ability to downregulate Cx43 mRNA, so nuclear localization was required for its function. We further identified a 3′ to 5′ exoribonuclease, RRP6 (ribosomal RNA processing protein 6), as a CELF1-interacting protein. The interaction of CELF1 and RRP6 was RNA-independent and nucleus specific. With knockdown of endogenous RRP6, CELF1 failed to downregulate Cx43 mRNA, which suggests that RRP6 was required for CELF1-mediated Cx43 mRNA degradation. In addition, increased CELF1 level accompanied upregulated RRP6, and reduced Cx43 level was detected in mouse models with DCM, including myotonic dystrophy type 1 and CELF1 overexpression models and a myocardial infarction model. Importantly, depletion of CELF1 in the infarcted heart preserved Cx43 mRNA level and ameliorated the cardiac phenotypes of the infarcted heart.

Conclusions:

Our results suggest a mechanism for increased CELF1 expression downregulating Cx43 mRNA level and a pathogenic role for elevated CELF1 level in the DCM heart.

Misregulation of calcium-handling proteins promotes hyperactivation of calcineurin-NFAT signaling in skeletal muscle of DM1 mice

Myotonic Dystrophy type 1 (DM1) is caused by an expansion of CUG repeats in DMPK mRNAs. This mutation affects alternative splicing through misregulation of RNA-binding proteins. Amongst pre-mRNAs that are mis-spliced, several code for proteins involved in calcium homeostasis suggesting that calcium-handling and signaling are perturbed in DM1. Here, we analyzed expression of such proteins in DM1 mouse muscle. We found that the levels of several sarcoplasmic reticulum proteins (SERCA1, sarcolipin and calsequestrin) are altered, likely contributing to an imbalance in calcium homeostasis. We also observed that calcineurin (CnA) signaling is hyperactivated in DM1 muscle. Indeed, CnA expression and phosphatase activity are both markedly increased in DM1 muscle. Coherent with this, we found that activators of the CnA pathway (MLP, FHL1) are also elevated. Consequently, NFATc1 expression is increased in DM1 muscle and becomes relocalized to myonuclei, together with an up-regulation of its transcriptional targets (RCAN1.4 and myoglobin). Accordingly, DM1 mouse muscles display an increase in oxidative metabolism and fiber hypertrophy. To determine the functional consequences of this CnA hyperactivation, we administered cyclosporine A, an inhibitor of CnA, to DM1 mice. Muscles of treated DM1 mice showed an increase in CUGBP1 levels, and an exacerbation of key alternative splicing events associated with DM1. Finally, inhibition of CnA in cultured human DM1 myoblasts also resulted in a splicing exacerbation of the insulin receptor. Together, these findings show for the first time that calcium-CnA signaling is hyperactivated in DM1 muscle and that such hyperactivation represents a beneficial compensatory adaptation to the disease.

Reduced cytoplasmic MBNL1 is an early event in a brain-specific mouse model of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is caused by an expansion of CTG repeats in the 3' untranslated region (UTR) of the dystrophia myotonia protein kinase (DMPK) gene. Cognitive impairment associated with structural change in the brain is prevalent in DM1. How this histopathological abnormality during disease progression develops remains elusive. Nuclear accumulation of mutant DMPK mRNA containing expanded CUG RNA disrupting the cytoplasmic and nuclear activities of muscleblind-like (MBNL) protein has been implicated in DM1 neural pathogenesis. The association between MBNL dysfunction and morphological changes has not been investigated. We generated a mouse model for postnatal expression of expanded CUG RNA in the brain that recapitulates the features of the DM1 brain, including the formation of nuclear RNA and MBNL foci, learning disability, brain atrophy and misregulated alternative splicing. Characterization of the pathological abnormalities by a time-course study revealed that hippocampus-related learning and synaptic potentiation were impaired before structural changes in the brain, followed by brain atrophy associated with progressive reduction of axon and dendrite integrity. Moreover, cytoplasmic MBNL1 distribution on dendrites decreased before dendrite degeneration, whereas reduced MBNL2 expression and altered MBNL-regulated alternative splicing was evident after degeneration. These results suggest that the expression of expanded CUG RNA in the DM1 brain results in neurodegenerative processes, with reduced cytoplasmic MBNL1 as an early event response to expanded CUG RNA.

Myotonic dystrophy: candidate small molecule therapeutics

Myotonic dystrophy type 1 (DM1) is a rare multisystemic neuromuscular disorder caused by expansion of CTG trinucleotide repeats in the noncoding region of the DMPK gene. Mutant DMPK transcripts are toxic and alter gene expression at several levels. Chiefly, the secondary structure formed by CUGs has a strong propensity to capture and retain proteins, like those of the muscleblind-like (MBNL) family. Sequestered MBNL proteins cannot then fulfill their normal functions. Many therapeutic approaches have been explored to reverse these pathological consequences. Here, we review the myriad of small molecules that have been proposed for DM1, including examples obtained from computational rational design, HTS, drug repurposing, and therapeutic gene modulation.

Myotonic dystrophy: approach to therapy

Myotonic dystrophy (DM) is a dominantly-inherited genetic disorder affecting skeletal muscle, heart, brain, and other organs. DM type 1 is caused by expansion of a CTG triplet repeat in DMPK, whereas DM type 2 is caused by expansion of a CCTG tetramer repeat in CNBP. In both cases the DM mutations lead to expression of dominant-acting RNAs. Studies of RNA toxicity have now revealed novel mechanisms and new therapeutic targets. Preclinical data have suggested that RNA dominance is responsive to therapeutic intervention and that DM therapy can be approached at several different levels. Here we review recent efforts to alleviate RNA toxicity in DM.

Myotonic Dystrophies: State of the Art of New Therapeutic Developments for the CNS

Myotonic dystrophies are multisystemic diseases characterized not only by muscle and heart dysfunction but also by CNS alteration. They are now recognized as brain diseases affecting newborns and children for myotonic dystrophy type 1 and adults for both myotonic dystrophy type 1 and type 2. In the past two decades, much progress has been made in understanding the mechanisms underlying the DM symptoms allowing development of new molecular therapeutic tools with the ultimate aim of curing the disease. This review describes the state of the art for the characterization of CNS related symptoms, the development of molecular strategies to target the CNS as well as the available tools for screening and testing new possible treatments.

Effect of PKC inhibitor on experimental autoimmune myocarditis in Lewis rats

Myocarditis is a major cause of sudden, unexpected death in young people. However, it is still one of the most challenging diseases to treat in cardiology. In the present study, we showed that both expression level and activity of PKC-α were up-regulated in the rat heart of experimental autoimmune myocarditis (EAM). Intraperitoneal administration of PKC inhibitor (Ro-32-0432) at the end of the most severe inflammation period of EAM still significantly reduced the EAM induced expression of failure biomarkers. Furthermore, Ro-32-0432 reduced the ratio of Bax/Bcl-2 and suppressed the expression of cleaved caspase-3, both of which were increased in the heart of the EAM rats, suggesting an anti-apoptotic role of Ro-32-0432. Besides, Ro-32-0432 suppressed EAM-induced cardiac fibrosis and release of pro-inflammatory cytokines IL-1β and IL-17. These results suggest that inhibition of PKC may serve as a potential therapeutic strategy for the treatment of myocarditis.

RNA-binding proteins, neural development and the addictions

Transcriptional and post-transcriptional regulation of gene expression defines the neurobiological mechanisms that bridge genetic and environmental risk factors with neurobehavioral dysfunction underlying the addictions. More than 1000 genes in the eukaryotic genome code for multifunctional RNA-binding proteins (RBPs) that can regulate all levels of RNA biogenesis. More than 50% of these RBPs are expressed in the brain where they regulate alternative splicing, transport, localization, stability and translation of RNAs during development and adulthood. Dysfunction of RBPs can exert global effects on their targetomes that underlie neurodegenerative disorders such as Alzheimer's and Parkinson's diseases as well as neurodevelopmental disorders, including autism and schizophrenia. Here, we consider the evidence that RBPs influence key molecular targets, neurodevelopment, synaptic plasticity and neurobehavioral dysfunction underlying the addictions. Increasingly well-powered genome-wide association studies in humans and mammalian model organisms combined with ever more precise transcriptomic and proteomic approaches will continue to uncover novel and possibly selective roles for RBPs in the addictions. Key challenges include identifying the biological functions of the dynamic RBP targetomes from specific cell types throughout subcellular space (e.g. the nuclear spliceome vs. the synaptic translatome) and time and manipulating RBP programs through post-transcriptional modifications to prevent or reverse aberrant neurodevelopment and plasticity underlying the addictions.

RNA-binding protein CELF1 promotes tumor growth and alters gene expression in oral squamous cell carcinoma

The RNA binding protein CELF1 (also known as CUGBP1) is emerging as a critical regulator of cancer cell proliferation and apoptosis. Here, to provide a global prospective of CELF1 regulation of oral squamous cell carcinoma, we performed RNA-sequencing in oral cancer cells and CELF1 overexpression analysis in non-malignant human oral keratinocytes. Our approaches identified 1283 mRNAs differentially regulated as a function of CELF1 expression and more importantly CELF1 promoted alternative splicing of several target pre-mRNAs, which are known to be involved in various cancer biological processes. Overexpression of CELF1 in non-malignant human oral keratinocytes protected cells against oxidative damage and altered gene expression patterns. Finally, we provide evidence that reduction of CELF1 protein using a xenograft tumorigenesis mouse model decreased tumor growth. Altogether, these data provided a comprehensive view of the CELF1 mRNA regulatory network in oral cancer and suggests that CELF1 and/or its target mRNAs are viable candidates for therapeutic intervention.

Disease Phenotypes in a Mouse Model of RNA Toxicity Are Independent of Protein Kinase Cα and Protein Kinase Cβ

Myotonic dystrophy type 1(DM1) is the prototype for diseases caused by RNA toxicity. RNAs from the mutant allele contain an expanded (CUG)_n tract within the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. The toxic RNAs affect the function of RNA binding proteins leading to sequestration of muscleblind-like (MBNL) proteins and increased levels of CELF1 (CUGBP, Elav-like family member 1). The mechanism for increased CELF1 is not very clear. One favored proposition is hyper-phosphorylation of CELF1 by Protein Kinase C alpha (PKCα) leading to increased CELF1 stability. However, most of the evidence supporting a role for PKC-α relies on pharmacological inhibition of PKC. To further investigate the role of PKCs in the pathogenesis of RNA toxicity, we generated transgenic mice with RNA toxicity that lacked both the PKCα and PKCβ isoforms. We find that these mice show similar disease progression as mice wildtype for the PKC isoforms. Additionally, the expression of CELF1 is also not affected by deficiency of PKCα and PKCβ in these RNA toxicity mice. These data suggest that disease phenotypes of these RNA toxicity mice are independent of PKCα and PKCβ.

Inositol 1,4,5-trisphosphate (IP3)-dependent Ca2+ signaling mediates delayed myogenesis in Duchenne muscular dystrophy fetal muscle

Duchenne muscular dystrophy (DMD) is a progressive neuromuscular disorder characterized by muscle wasting and premature death. The defective gene is dystrophin, a structural protein, absence of which causes membrane fragility and myofiber necrosis. Several lines of evidence showed that in adult DMD patients dystrophin is involved in signaling pathways that regulate calcium homeostasis and differentiation programs. However, secondary aspects of the disease, such as inflammation and fibrosis development, might represent a bias in the analysis. Because fetal muscle is not influenced by gravity and does not suffer from mechanical load and/or inflammation, we investigated 12-week-old fetal DMD skeletal muscles, highlighting for the first time early alterations in signaling pathways mediated by the absence of dystrophin itself. We found that PLC/IP3/IP3R/Ryr1/Ca2+ signaling is widely active in fetal DMD skeletal muscles and, through the calcium-dependent PKCα protein, exerts a fundamental regulatory role in delaying myogenesis and in myofiber commitment. These data provide new insights into the origin of DMD pathology during muscle development.

Feedback Regulation of Kinase Signaling Pathways by AREs and GREs

In response to environmental signals, kinases phosphorylate numerous proteins, including RNA-binding proteins such as the AU-rich element (ARE) binding proteins, and the GU-rich element (GRE) binding proteins. Posttranslational modifications of these proteins lead to a significant changes in the abundance of target mRNAs, and affect gene expression during cellular activation, proliferation, and stress responses. In this review, we summarize the effect of phosphorylation on the function of ARE-binding proteins ZFP36 and ELAVL1 and the GRE-binding protein CELF1. The networks of target mRNAs that these proteins bind and regulate include transcripts encoding kinases and kinase signaling pathways (KSP) components. Thus, kinase signaling pathways are involved in feedback regulation, whereby kinases regulate RNA-binding proteins that subsequently regulate mRNA stability of ARE- or GRE-containing transcripts that encode components of KSP.

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.

Myotonic dystrophy

Myotonic dystrophy (dystrophia myotonica, DM) is one of the most common lethal monogenic disorders in populations of European descent. DM type 1 was first described over a century ago. More recently, a second form of the disease, DM type 2 was recognized, which results from repeat expansion in a different gene. Both disorders have autosomal dominant inheritance and multisystem features, including myotonic myopathy, cataract, and cardiac conduction disease. This article reviews the clinical presentation and pathophysiology of DM and discusses current management and future potential for developing targeted therapies.

LDB3 splicing abnormalities are specific to skeletal muscles of patients with myotonic dystrophy type 1 and alter its PKC binding affinity

Myotonic dystrophy type 1 (DM1) is caused by transcription of CUG repeat RNA, which causes sequestration of muscleblind-like 1 (MBNL1) and upregulation of CUG triplet repeat RNA-binding protein (CUG-BP1). In DM1, dysregulation of these proteins contributes to many aberrant splicing events, causing various symptoms of the disorder. Here, we demonstrate the occurrence of aberrant splicing of LIM domain binding 3 (LDB3) exon 11 in DM1 skeletal muscle. Exon array surveys, RT-PCR, and western blotting studies demonstrated that exon 11 inclusion was DM1 specific and could be reproduced by transfection of a minigene containing the CTG repeat expansion. Moreover, we found that the LDB3 exon 11-positive isoform had reduced affinity for PKC compared to the exon 11-negative isoform. Since PKC exhibits hyperactivation in DM1 and stabilizes CUG-BP1 by phosphorylation, aberrant splicing of LDB3 may contribute to CUG-BP1 upregulation through changes in its affinity for PKC.

Small molecule kinase inhibitors alleviate different molecular features of myotonic dystrophy type 1

Expandable (CTG)n repeats in the 3′ UTR of the DMPK gene are a cause of myotonic dystrophy type 1 (DM1), which leads to a toxic RNA gain-of-function disease. Mutant RNAs with expanded CUG repeats are retained in the nucleus and aggregate in discrete inclusions. These foci sequester splicing factors of the MBNL family and trigger upregulation of the CUGBP family of proteins resulting in the mis-splicing of their target transcripts. To date, many efforts to develop novel therapeutic strategies have been focused on disrupting the toxic nuclear foci and correcting aberrant alternative splicing via targeting mutant CUG repeats RNA; however, no effective treatment for DM1 is currently available. Herein, we present results of culturing of human DM1 myoblasts and fibroblasts with two small-molecule ATP-binding site-specific kinase inhibitors, C16 and C51, which resulted in the alleviation of the dominant-negative effects of CUG repeat expansion. Reversal of the DM1 molecular phenotype includes a reduction of the size and number of foci containing expanded CUG repeat transcripts, decreased steady-state levels of CUGBP1 protein, and consequent improvement of the aberrant alternative splicing of several pre-mRNAs misregulated in DM1.

Mouse models of heart failure: cell signaling and cell survival

Heart failure is one of the paramount global causes of morbidity and mortality. Despite this pandemic need, the available clinical counter-measures have not altered substantially in recent decades, most notably in the context of pharmacological interventions. Cell death plays a causal role in heart failure, and its inhibition poses a promising approach that has not been thoroughly explored. In previous approaches to target discovery, clinical failures have reflected a deficiency in mechanistic understanding, and in some instances, failure to systematically translate laboratory findings toward the clinic. Here, we review diverse mouse models of heart failure, with an emphasis on those that identify potential targets for pharmacological inhibition of cell death, and on how their translation into effective therapies might be improved in the future.

RNA-binding proteins in heart development

RNA-binding proteins (RBPs) are key players of posttranscriptional regulation occurring during normal tissue development. All tissues examined thus far have revealed the importance of RBPs in the regulation of complex networks involved in organ morphogenesis, maturation, and function. They are responsible for controlling tissue-specific gene expression by regulating alternative splicing, mRNA stability, translation, and poly-adenylation. The heart is the first organ form during embryonic development and is also the first to acquire functionality. Numerous remodeling processes take place during late cardiac development since fetal heart first adapts to birth and then undergoes a transition to adult functionality. This physiological remodeling involves transcriptional and posttranscriptional networks that are regulated by RBPs. Disruption of the normal regulatory networks has been shown to cause cardiomyopathy in humans and animal models. Here we review the complexity of late heart development and the current information regarding how RBPs control aspects of postnatal heart development. We also review how activities of RBPs are modulated adding complexity to the regulation of developmental networks.

High-content screening identifies small molecules that remove nuclear foci, affect MBNL distribution and CELF1 protein levels via a PKC-independent pathway in myotonic dystrophy cell lines

Myotonic dystrophy (DM) is a multi-system neuromuscular disorder for which there is no treatment. We have developed a medium throughput phenotypic assay, based on the identification of nuclear foci in DM patient cell lines using in situ hybridization and high-content imaging to screen for potentially useful therapeutic compounds. A series of further assays based on molecular features of DM have also been employed. Two compounds that reduce and/or remove nuclear foci have been identified, Ro 31-8220 and chromomycin A3. Ro 31-8220 is a PKC inhibitor, previously shown to affect the hyperphosphorylation of CELF1 and ameliorate the cardiac phenotype in a DM1 mouse model. We show that the same compound eliminates nuclear foci, reduces MBNL1 protein in the nucleus, affects ATP2A1 alternative splicing and reduces steady-state levels of CELF1 protein. We demonstrate that this effect is independent of PKC activity and conclude that this compound may be acting on alternative kinase targets within DM pathophysiology. Understanding the activity profile for this compound is key for the development of targeted therapeutics in the treatment of DM.

Mechanisms of altered Ca²⁺ handling in heart failure

Ca²⁺ plays a crucial role in connecting membrane excitability with contraction in myocardium. The hallmark features of heart failure are mechanical dysfunction and arrhythmias; defective intracellular Ca²⁺ homeostasis is a central cause of contractile dysfunction and arrhythmias in failing myocardium. Defective Ca²⁺ homeostasis in heart failure can result from pathological alteration in the expression and activity of an increasingly understood collection of Ca²⁺ homeostatic and structural proteins, ion channels, and enzymes. This review focuses on the molecular mechanisms of defective Ca²⁺ cycling in heart failure and considers how fundamental understanding of these pathways may translate into novel and innovative therapies.

Reactivation of fetal splicing programs in diabetic hearts is mediated by protein kinase C signaling

Diabetic cardiomyopathy is one of the complications of diabetes that eventually leads to heart failure and death. Aberrant activation of PKC signaling contributes to diabetic cardiomyopathy by mechanisms that are poorly understood. Previous reports indicate that PKC is implicated in alternative splicing regulation. Therefore, we wanted to test whether PKC activation in diabetic hearts induces alternative splicing abnormalities. Here, using RNA sequencing we identified a set of 22 alternative splicing events that undergo a developmental switch in splicing, and we confirmed that splicing reverts to an embryonic pattern in adult diabetic hearts. This network of genes has important functions in RNA metabolism and in developmental processes such as differentiation. Importantly, PKC isozymes α/β control alternative splicing of these genes via phosphorylation and up-regulation of the RNA-binding proteins CELF1 and Rbfox2. Using a mutant of CELF1, we show that phosphorylation of CELF1 by PKC is necessary for regulation of splicing events altered in diabetes. In summary, our studies indicate that activation of PKCα/β in diabetic hearts contributes to the genome-wide splicing changes through phosphorylation and up-regulation of CELF1/Rbfox2 proteins. These findings provide a basis for PKC-mediated cardiac pathogenesis under diabetic conditions.

Manumycin A corrects aberrant splicing of Clcn1 in myotonic dystrophy type 1 (DM1) mice

Myotonic dystrophy type 1 (DM1) is the most common muscular dystrophy in adults and as yet no cure for DM1. Here, we report the potential of manumycin A for a novel DM1 therapeutic reagent. DM1 is caused by expansion of CTG repeat. Mutant transcripts containing expanded CUG repeats lead to aberrant regulation of alternative splicing. Myotonia (delayed muscle relaxation) is the most commonly observed symptom in DM1 patients and is caused by aberrant splicing of the skeletal muscle chloride channel (CLCN1) gene. Identification of small-molecule compounds that correct aberrant splicing in DM1 is attracting much attention as a way of improving understanding of the mechanism of DM1 pathology and improving treatment of DM1 patients. In this study, we generated a reporter screening system and searched for small-molecule compounds. We found that manumycin A corrects aberrant splicing of Clcn1 in cell and mouse models of DM1.

Aberrant splicing in neurological diseases

Splicing of precursor messenger RNA (pre-mRNA) removes the intervening sequences (introns) and joins the expressed regions (exons) in the nucleus, before an intron-containing eukaryotic mRNA transcript can be exported and translated into proteins in the cytoplasm. While some sequences are always included or excluded (constitutive splicing), others can be selectively used (alternative splicing) in this process. Particularly by alternative splicing, up to tens of thousands of variant transcripts can be produced from a single gene, which contributes greatly to the proteomic diversity for such complex cellular functions as 'wiring' neurons in the nervous system. Disruption of this process leads to aberrant splicing, which accounts for the defects of up to 50% of mutations that cause certain human genetic diseases. In this review, we describe the different mechanisms of aberrant splicing that cause or have been associated with neurological diseases.

Molecular mechanisms of muscle atrophy in myotonic dystrophies

Myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2) are multisystemic diseases that primarily affect skeletal muscle, causing myotonia, muscle atrophy, and muscle weakness. DM1 and DM2 pathologies are caused by expansion of CTG and CCTG repeats in non-coding regions of the genes encoding myotonic dystrophy protein kinase (DMPK) and zinc finger protein 9 (ZNF9) respectively. These expansions cause DM pathologies through accumulation of mutant RNAs that alter RNA metabolism in patients' tissues by targeting RNA-binding proteins such as CUG-binding protein 1 (CUGBP1) and Muscle blind-like protein 1 (MBNL1). Despite overwhelming evidence showing the critical role of RNA-binding proteins in DM1 and DM2 pathologies, the downstream pathways by which these RNA-binding proteins cause muscle wasting and muscle weakness are not well understood. This review discusses the molecular pathways by which DM1 and DM2 mutations might cause muscle atrophy and describes progress toward the development of therapeutic interventions for muscle wasting and weakness in DM1 and DM2. This article is part of a Directed Issue entitled: Molecular basis of muscle wasting.