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

The alternative heart: impact of alternative splicing in heart disease

Alternative splicing is the main driver of protein diversity and allows the production of different proteins from each gene in the genome. Changes in exon exclusion, intron retention or the use of alternative splice sites can alter protein structure, localisation, regulation and function. In the heart, alternative splicing of sarcomeric genes, ion channels and cell signalling proteins can lead to cardiomyopathies, arrhythmias and other pathologies. Also, a number of inherited conditions and heart-related diseases develop as a result of mutations affecting splicing. Here, we review the impact that changes in alternative splicing have on individual genes and on whole biological processes associated with heart disease. We also discuss promising therapeutic tools based on the manipulation of alternative splicing.

Smaug/SAMD4A restores translational activity of CUGBP1 and suppresses CUG-induced myopathy

We report the identification and characterization of a previously unknown suppressor of myopathy caused by expansion of CUG repeats, the mutation that triggers Myotonic Dystrophy Type 1 (DM1). We screened a collection of genes encoding RNA-binding proteins as candidates to modify DM1 pathogenesis using a well established Drosophila model of the disease. The screen revealed smaug as a powerful modulator of CUG-induced toxicity. Increasing smaug levels prevents muscle wasting and restores muscle function, while reducing its function exacerbates CUG-induced phenotypes. Using human myoblasts, we show physical interactions between human Smaug (SMAUG1/SMAD4A) and CUGBP1. Increased levels of SMAUG1 correct the abnormally high nuclear accumulation of CUGBP1 in myoblasts from DM1 patients. In addition, augmenting SMAUG1 levels leads to a reduction of inactive CUGBP1-eIF2α translational complexes and to a correction of translation of MRG15, a downstream target of CUGBP1. Therefore, Smaug suppresses CUG-mediated muscle wasting at least in part via restoration of translational activity of CUGBP1.

RNA toxicity in human disease and animal models: from the uncovering of a new mechanism to the development of promising therapies

Mutant ribonucleic acid (RNA) molecules can be toxic to the cell, causing human disease through trans-acting dominant mechanisms. RNA toxicity was first described in myotonic dystrophy type 1, a multisystemic disorder caused by the abnormal expansion of a non-coding trinucleotide repeat sequence. The development of multiple and complementary animal models of disease has greatly contributed to clarifying the complex disease pathways mediated by toxic RNA molecules. RNA toxicity is not limited to myotonic dystrophy and spreads to an increasing number of human conditions, which share some unifying pathogenic events mediated by toxic RNA accumulation and disruption of RNA-binding proteins. The remarkable progress in the dissection of disease pathobiology resulted in the rational design of molecular therapies, which have been successfully tested in animal models. Toxic RNA diseases, and in particular myotonic dystrophy, clearly illustrate the critical contribution of animal models of disease in translational research: from gene mutation to disease mechanisms, and ultimately to therapy development. This article is part of a Special Issue entitled: Animal Models of Disease.

Antisense oligonucleotides: rising stars in eliminating RNA toxicity in myotonic dystrophy

Myotonic dystrophy (DM) is a dominantly inherited, multisystemic disease caused by expanded CTG (type 1, DM1) or CCTG (type 2, DM2) repeats in untranslated regions of the mutated genes. Pathogenesis results from expression of RNAs from the mutated alleles that are toxic because of the expanded CUG or CCUG repeats. Increased understanding of the repeat-containing RNA (C/CUG(exp) RNA)-induced toxicity has led to the development of multiple strategies targeting the toxic RNA. Among these approaches, antisense oligonucleotides (ASOs) have demonstrated high potency in reversing the RNA toxicity in both cultured DM1 cells and DM1 animal models, thus offering great promise for the potential treatment of DM1. ASO targeting approaches will also provide avenues for the treatment of other repeat RNA-mediated diseases.

CUG-binding protein 1 (CUGBP1) expression and prognosis of non-small cell lung cancer

BACKGROUND AND AIMS

Non-small cell lung cancer (NSCLC) is the leading cause of cancer mortality worldwide. As CUGBP1 may also play a great role in tumor genesis and deterioration, the purpose of this study was to detect the expression of CUGBP1 mRNA and CUGBP1 and assess the prognostic significance of CUGBP1 in NSCLC.

METHODS

Expression of CUGBP1 mRNA and CUGBP was detected by Semi-quantitative PCR and Immunohistochemistry, respectively, from 57 NSCLC patients. The percentage of CUGBP1 mRNA and CUGBP1 expression was correlated with clinical characteristics using χ (2) test. The prognostic significance was assessed by univariate and multivariate analyses in the Cox hazard model.

RESULTS

The expression of CUGBP1 mRNA and CUGBP1 was over-expressed in cancer group and was correlated with TNM stage and Differentiation. By both univariate and multivariate survival analyses, CUGBP1 expression (P = 0.0074, HR = 3.701, 95 % CI 1.420-9.648), TNM-stage (HR = 4.043, 95 % CI 2.098-7.794) and age (HR = 3.207, 95 % CI 1.544-6.664) were noted to be independent indicators of a shorter postsurgical survival.

CONCLUSIONS

The expression of CUGBP1 independently predicted a shorter postsurgical survival in NSCLC.

CELFish ways to modulate mRNA decay

The CELF family of RNA-binding proteins regulates many steps of mRNA metabolism. Although their best characterized function is in pre-mRNA splice site choice, CELF family members are also powerful modulators of mRNA decay. In this review we focus on the different modes of regulation that CELF proteins employ to mediate mRNA decay by binding to GU-rich elements. After starting with an overview of the importance of CELF proteins during development and disease pathogenesis, we then review the mRNA networks and cellular pathways these proteins regulate and the mechanisms by which they influence mRNA decay. Finally, we discuss how CELF protein activity is modulated during development and in response to cellular signals. We conclude by highlighting the priorities for new experiments in this field. This article is part of a Special Issue entitled: RNA Decay mechanisms.

Signaling effectors underlying pathologic growth and remodeling of the heart

Cardiovascular disease is the number one cause of mortality in the Western world. The heart responds to many cardiopathological conditions with hypertrophic growth by enlarging individual myocytes to augment cardiac pump function and decrease ventricular wall tension. Initially, such cardiac hypertrophic growth is often compensatory, but as time progresses these changes become maladaptive. Cardiac hypertrophy is the strongest predictor for the development of heart failure, arrhythmia, and sudden death. Here we discuss therapeutic avenues emerging from molecular and genetic studies of cardiovascular disease in animal models. The majority of these are based on intracellular signaling pathways considered central to pathologic cardiac remodeling and hypertrophy, which then leads to heart failure. We focus our discussion on selected therapeutic targets that have more recently emerged and have a tangible translational potential given the available pharmacologic agents that could be readily evaluated in human clinical trials.

CUG-BP, Elav-like family (CELF)-mediated alternative splicing regulation in the brain during health and disease

Alternative splicing is an important mechanism for generating transcript and protein diversity. In the brain, alternative splicing is particularly prevalent, and alternative splicing factors are highly enriched. These include the six members of the CUG-BP, Elav-like family (CELF). This review summarizes what is known about the expression of different CELF proteins in the nervous system and the evidence that they are important in neural development and function. The involvement of CELF proteins in the pathogenesis of a number of neurodegenerative disorders, including myotonic dystrophy, spinocerebellar ataxia, fragile X syndrome, spinal muscular atrophy, and spinal and bulbar muscular atrophy is discussed. Finally, the known targets of CELF-mediated alternative splicing regulation in the nervous system and the functional consequences of these splicing events are reviewed. This article is part of a Special Issue entitled "RNA and splicing regulation in neurodegeneration."

Protein kinase Cα is involved in impaired perinatal hypothyroid rat brain development

Protein kinase Cα (PKCα) has been implicated in the regulation of a variety of cellular functions, such as proliferation, differentiation, and apoptosis, in response to a diverse range of stimuli. Activated PKCα mediates oxidative stress, apoptosis, and inflammatory reaction. Thyroid hormone (TH) is essential for the proper development of the mammalian central nervous system. TH deficiency during critical periods of brain development results in permanent cognitive and neurological impairments. In the present study, we attempted to explore whether PKCα is involved in impaired brain function in developing hypothyroid rat brain. Severe perinatal hypothyroidism was obtained by administration of 30 mg/day propylthiouracil to dams. Brain PKC activity in hypothyroid pups was increased significantly in cytosol and membrane fractions. The change of membrane PKC activity was more marked than that of cytosol, and hypothyroidism led to a higher ratio of membrane PKC activity to that in cytosol, which means abnormal activation of PKC in developing hypothyroid rat brain. Thyroxine replacement partially corrected these changes. After being treated with bisindolmaleimide XI, a mainly selective inhibitor for PKCα, the hypothyroid pups showed improved place navigation test results, and further Western blot analysis showed that PKCα expression in cytosol fractions was increased in hypothyroid rat brain with or without bisindolmaleimide XI treatment, but, after treatment with bisindolmaleimide XI, PKCα content in membrane fractions decreased almost to normal. Therefore, we conclude that PKCα appears to be involved in the impaired brain development observed in perinatal hypothyroid rat brain.

Regulation of the alternative splicing of sarcoplasmic reticulum Ca²⁺-ATPase1 (SERCA1) by phorbol 12-myristate 13-acetate (PMA) via a PKC pathway

Myotonic dystrophy type 1 (DM1) is a multi-systemic disease with no established treatment to date. Small, cell-permeable molecules hold the potential to treat DM1. In this study, we investigated the association between protein kinase C (PKC) signaling and splicing of sarcoplasmic reticulum Ca(2+)-ATPase1 (SERCA1). Our aim was to clarify the mechanisms underlying the regulation of alternative splicing, in order to explore new therapeutic strategies for DM1. By assessing the splicing pattern of the endogenous SERCA1 gene in HEK293 cells, we found that treatment with phorbol 12-myristate 13-acetate (PMA) regulated SERCA1 splicing. Interestingly, treatment with PMA for 48 h normalized SERCA1 splicing, while treatment for 1.5h promoted aberrant splicing. These two responses showed dose dependency and were completely abolished by the PKC inhibitor Ro 31-8220. Furthermore, repression of PKCβII and PKCθ by RNAi mimicked prolonged PMA treatment. These results indicate that PKC signaling is involved in the splicing of SERCA1 and provide new evidence for a link between alternative splicing and PKC signaling.

Repression of nuclear CELF activity can rescue CELF-regulated alternative splicing defects in skeletal muscle models of myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is caused by the expansion of CUG repeats in the 3’ UTR of DMPK transcripts. DM1 pathogenesis has been attributed in part to alternative splicing dysregulation via elevation of CUG-BP, Elav-like family member 1 (CELF1). Several therapeutic approaches have been tested in cells and mice, but no previous studies had specifically targeted CELF1. Here, we show that repressing CELF activity rescues CELF-dependent alternative splicing in cell culture and transgenic mouse models of DM1. CELF-independent splicing, however, remained dysregulated. These data highlight both the potential and limitations of targeting CELF1 for the treatment of DM1.

Role of noncoding RNAs in trinucleotide repeat neurodegenerative disorders

Increasingly complex networks of noncoding RNAs are being found to play important and diverse roles in the regulation of gene expression throughout the genome. Many lines of evidence are linking mutations and dysregulations of noncoding RNAs to a host of human diseases, and noncoding RNAs have been implicated in the molecular pathogenesis of some neurodegenerative disorders. The expansion of trinucleotide repeats is now recognized as a major cause of neurological disorders. Here we will review our current knowledge of the proposed mechanisms behind the involvement of noncoding RNAs in the molecular pathogenesis of neurodegenerative disorders, particularly the sequestration of specific RNA-binding proteins, the regulation of antisense transcripts, and the role of the microRNA pathway in the context of known neurodegenerative disorders caused by the expansion of trinucleotide repeats.

Regulation of CUG-binding protein 1 (CUGBP1) binding to target transcripts upon T cell activation

The RNA-binding protein, CUG-binding protein 1 (CUGBP1), regulates gene expression at the levels of alternative splicing, mRNA degradation, and translation. We used RNA immunoprecipitation followed by microarray analysis to identify the cytoplasmic mRNA targets of CUGBP1 in resting and activated primary human T cells and found that CUGBP1 targets were highly enriched for the presence of GU-rich elements (GREs) in their 3'-untranslated regions. The number of CUGBP1 target transcripts decreased dramatically following T cell activation as a result of activation-dependent phosphorylation of CUGBP1 and decreased ability of CUGBP1 to bind to GRE-containing RNA. A large percentage of CUGBP1 target transcripts exhibited rapid and transient up-regulation, and a smaller percentage exhibited transient down-regulation following T cell activation. Many of the transiently up-regulated CUGBP1 target transcripts encode important regulatory proteins necessary for transition from a quiescent state to a state of cellular activation and proliferation. Overall, our results show that CUGBP1 binding to certain GRE-containing target transcripts decreased following T cell activation through activation-dependent phosphorylation of CUGBP1.

Inhibition of PKCα/β with ruboxistaurin antagonizes heart failure in pigs after myocardial infarction injury

RATIONALE

Protein kinase Cα (PKCα) activity and protein level are induced during cardiac disease where it controls myocardial contractility and propensity to heart failure in mice and rats. For example, mice lacking the gene for PKCα have enhanced cardiac contractility and reduced susceptibility to heart failure after long-term pressure overload or after myocardial infarction injury. Pharmacological inhibition of PKCα/β with Ro-32-0432, Ro-31-8220 or ruboxistaurin (LY333531) similarly enhances cardiac function and antagonizes heart failure in multiple models of disease in both mice and rats.

OBJECTIVE

Large and small mammals differ in several key indexes of heart function and biochemistry, lending uncertainty as to how PKCα/β inhibition might affect or protect a large animal model of heart failure.

METHODS AND RESULTS

We demonstrate that ruboxistaurin administration to a pig model of myocardial infarction-induced heart failure was protective. Twenty-kilogram pigs underwent left anterior descending artery occlusion resulting in myocardial infarctions and were then divided into vehicle or ruboxistaurin feed groups, after which they were monitored monthly for the next 3 months. Ruboxistaurin administered pigs showed significantly better recovery of myocardial contractility 3 months after infarction injury, greater ejection fraction, and greater cardiac output compared with vehicle-treated pigs.

CONCLUSIONS

These results provide additional evidence in a large animal model of disease that PKCα/β inhibition (with ruboxistaurin) represents a tenable and novel therapeutic approach for treating human heart failure.

Protein kinase Cα as a heart failure therapeutic target

Heart failure afflicts ~5 million people and causes ~300,000 deaths a year in the United States alone. Heart failure is defined as a deficiency in the ability of the heart to pump sufficient blood in response to systemic demands, which results in fatigue, dyspnea, and/or edema. Identifying new therapeutic targets is a major focus of current research in the field. We and others have identified critical roles for protein kinase C (PKC) family members in programming aspects of heart failure pathogenesis. More specifically, mechanistic data have emerged over the past 6-7 years that directly implicate PKCα, a conventional PKC family member, as a nodal regulator of heart failure propensity. Indeed, deletion of the PKCα gene in mice, or its inhibition in rodents with drugs or a dominant negative mutant and/or inhibitory peptide, has shown dramatic protective effects that antagonize the development of heart failure. This review will weigh all the evidence implicating PKCα as a novel therapeutic target to consider for the treatment of heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."

Targeting RNA to treat neuromuscular disease

The development of effective therapies for neuromuscular disorders such as Duchenne muscular dystrophy (DMD) is hampered by considerable challenges: skeletal muscle is the most abundant tissue in the body, and many neuromuscular disorders are multisystemic conditions. However, despite these barriers there has recently been substantial progress in the search for novel treatments. In particular, the use of antisense oligonucleotides, which are designed to target RNA and modulate pre-mRNA splicing to restore functional protein isoforms or directly inhibit the toxic effects of pathogenic RNAs, offers great promise and these approaches are now being tested in the clinic. Here, we review recent advances in the development of such antisense oligonucleotides and other promising novel approaches, including the induction of readthrough nonsense mutations.

Gain of RNA function in pathological cases: Focus on myotonic dystrophy

Expansion of repeated sequences in non-coding regions of different genes causes a number of inherited diseases including myotonic dystrophies, Huntington disease-like 2, Fragile X tremor/ataxia syndrome and spinocerebellar ataxia 8, 10, 12, 31. Involvement of an RNA gain-of-function mechanism in pathological case has been described and studied in-depth in myotonic dystrophy type 1 (DM1). This inherited neuromuscular disorder is caused by a (CTG)n >50 expansion in the 3' non-coding region of the dystrophia myotonica-protein kinase (DMPK) gene. Expanded CUG transcripts (CUGexp-RNAs) are sequestered in the nucleus within small aggregates and interfere with the regulatory splicing activities of MBNL1 and CELF1 RNA-binding proteins, leading to the misregulation of the alternative splicing of several transcripts. Despite the relevance of aberrant splicing events in this complex pathology, the CUGexp-RNAs trans-dominant effects alter other splicing-independent processes that may also contribute to DM1 pathogenesis. This review will focus on toxic RNA gain-of-function as a pathologic mechanism for DM1 and other repeat expansion disorders.

Myotonic dystrophy mouse models: towards rational therapy development

DNA repeat expansions can result in the production of toxic RNA. RNA toxicity has been best characterised in the context of myotonic dystrophy. Nearly 20 mouse models have contributed significant and complementary insights into specific aspects of this novel disease mechanism. These models provide a unique resource to test pharmacological, anti-sense, and gene-therapy therapeutic strategies that target specific events of the pathobiological cascade. Further proof-of-principle concept studies and preclinical experiments require critical and thorough analysis of the multiple myotonic dystrophy transgenic lines available. This review provides in-depth assessment of the molecular and phenotypic features of these models and their contribution towards the dissection of disease mechanisms, and compares them with the human condition. More importantly, it provides critical assessment of their suitability and limitations for preclinical testing of emerging therapeutic strategies.

Therapeutics development in myotonic dystrophy type 1

Myotonic dystrophy (DM1), the most common adult muscular dystrophy, is a multisystem, autosomal dominant genetic disorder caused by an expanded CTG repeat that leads to nuclear retention of a mutant RNA and subsequent RNA toxicity. Significant insights into the molecular mechanisms of RNA toxicity have led to the previously unforeseen possibility that treating DM1 is a viable prospect. In this review, we briefly present the clinical picture in DM1, and describe how the research in understanding the pathogenesis of RNA toxicity in DM1 has led to targeted approaches to therapeutic development at various steps in the pathogenesis of the disease. We discuss the promise and current limitations of each with an emphasis on RNA-based therapeutics and small molecules. We conclude with a discussion of the unmet need for clinical tools and outcome measures that are essential prerequisites to proceed in evaluating these potential therapies in clinical trials.

Pip5 transduction peptides direct high efficiency oligonucleotide-mediated dystrophin exon skipping in heart and phenotypic correction in mdx mice

Induced splice modulation of pre-mRNAs shows promise to correct aberrant disease transcripts and restore functional protein and thus has therapeutic potential. Duchenne muscular dystrophy (DMD) results from mutations that disrupt the DMD gene open reading frame causing an absence of dystrophin protein. Antisense oligonucleotide (AO)-mediated exon skipping has been shown to restore functional dystrophin in mdx mice and DMD patients treated intramuscularly in two recent phase 1 clinical trials. Critical to the therapeutic success of AO-based treatment will be the ability to deliver AOs systemically to all affected tissues including the heart. Here, we report identification of a series of transduction peptides (Pip5) as AO conjugates for enhanced systemic and particularly cardiac delivery. One of the lead peptide-AO conjugates, Pip5e-AO, showed highly efficient exon skipping and dystrophin production in mdx mice with complete correction of the aberrant DMD transcript in heart, leading to >50% of the normal level of dystrophin in heart. Mechanistic studies indicated that the enhanced activity of Pip5e-phosphorodiamidate morpholino (PMO) is partly explained by more efficient nuclear delivery. Pip5 series derivatives therefore have significant potential for advancing the development of exon skipping therapies for DMD and may have application for enhanced cardiac delivery of other biotherapeutics.

Altered signal transduction pathways and induction of autophagy in human myotonic dystrophy type 1 myoblasts

Congenital myotonic dystrophy type 1 (CDM1) affects patients from birth and is associated with mental retardation and impaired muscle development. CDM1 patients carry 1000-3000 CTG repeats in the DMPK gene and display defective skeletal muscles differentiation, resulting in reduced size of myotubes and decreased number of satellite cells. In this study, human myoblasts in culture deriving from control and DM1 embryos (3200 CTG repeats) were analyzed using both a biochemical and electron microscopic approach, in order to provide new insights into the molecular mechanisms underlying such alteration. Interestingly, electron microscopy analysis showed not only ultrastructural features of abnormal differentiation but also revealed the presence of autophagic vacuoles in DM1 myoblasts not undergoing differentiation. In accordance with the electron microscopic findings, the autophagic markers LC3 and ATG5, but not apoptotic markers, were significantly up regulated in DM1 myoblasts after differentiating medium addition. The induction of autophagic processes in DM1 myoblasts was concomitant to p53 over-expression and inhibition of the mTOR-S6K1 pathway, causatively involved in autophagy. Moreover biochemical alterations of the two main signal transduction pathways involved in differentiation were observed in DM1 myoblasts, in particular decreased activation of p38MAPK and persistent activation of the MEK-ERK pathway. This work, while demonstrating that major signaling pathways regulating myoblasts differentiation are profoundly deranged in DM1 myoblasts, for the first time provides evidence of autophagy induction, possibly mediated by p53 activation in response to metabolic stress which might contribute to the dystrophic alterations observed in the muscles of congenital DM1 patients.

Tackling the pathogenesis of RNA nuclear retention in myotonic dystrophy

DM1 (myotonic dystrophy type I) is a common form of muscular dystrophy that affects mainly adults. It is a disease that belongs to the group of defective RNA export diseases, since a major part of the pathogenic mechanism of the disease is the retention of the mutant transcripts in the cell nucleus. The presence of an expanded CUG trinucleotide repeat in the 3'-UTR (3'-untranslated region) of the DMPK (myotonic dystrophy protein kinase) gene causes the attraction of RNA-binding proteins by the nuclear-located mutant transcripts. As a result of the occupation of the RNA-binding proteins, there is defective mis-splicing of several cellular transcripts. This is believed to be a major pathogenic mechanism of the disease and any attempt to repair the activities of the RNA-binding proteins or target the mutant transcripts should be beneficial for the patients. Certain approaches have been described in the literature and they demonstrate progress in various directions. The purpose of the present review is to summarize the successful attempts to tackle the pathogenesis caused by nuclear retention of mutant transcripts in myotonic dystrophy and to discuss the possible gains from such approaches.

CUGBP1 overexpression in mouse skeletal muscle reproduces features of myotonic dystrophy type 1

The neuromuscular disease myotonic dystrophy type I (DM1) affects multiple organ systems with the major symptoms being severe muscle weakness, progressive muscle wasting and myotonia. The causative mutation in DM1 is a CTG repeat expansion in the 3'-untranslated region of the DM protein kinase (DMPK) gene. RNA transcribed from the expanded allele contains the expanded CUG repeats and leads to the nuclear depletion of Muscleblind-like 1 (MBNL1) and to the increased steady-state levels of CUG-binding protein 1 (CUGBP1). The pathogenic effects of MBNL1 depletion have previously been tested by the generation of MBNL1 knockout mice, but the consequence of CUGBP1 overexpression in adult muscle is not known. In a DM1 mouse model expressing RNA containing 960 CUG repeats in skeletal muscle, CUGBP1 up-regulation is temporally correlated with severe muscle wasting. In this study, we generated transgenic mice with doxycycline-inducible and skeletal muscle-specific expression of CUGBP1. Adult mouse skeletal muscle overexpressing CUGBP1 reproduces molecular and physiological defects of DM1 tissue. The results from this study strongly suggest that CUGBP1 has a major role in DM1 skeletal muscle pathogenesis.

Repeat expansion diseases: when a good RNA turns bad

An increasing number of dominantly inherited diseases have now been linked with expansion of short repeats within specific genes. Although some of these expansions affect protein function or result in haploinsufficiency, a significant portion cause pathogenesis through production of toxic RNA molecules that alter cellular metabolism. In this review, we examine the criteria that influence toxicity of these mutant RNAs and discuss new developments in therapeutic approaches.