Reduction of the rate of protein translation in patients with myotonic dystrophy 2

A KIF1C-CNBP motor-adaptor complex for trafficking mRNAs to cell protrusions

mRNA localization to subcellular compartments is a widely used mechanism that functionally contributes to numerous processes. mRNA targeting can be achieved upon recognition of RNA cargo by molecular motors. However, our molecular understanding of how this is accomplished is limited, especially in higher organisms. We focus on a pathway that targets mRNAs to peripheral protrusions of mammalian cells and which is important for cell migration. Trafficking occurs through active transport on microtubules, mediated by the KIF1C kinesin. Here, we identify the RNA-binding protein CNBP as a factor required for mRNA localization to protrusions. CNBP binds directly to GA-rich sequences in the 3' UTR of protrusion-targeted mRNAs. CNBP also interacts with KIF1C and is required for KIF1C recruitment to mRNAs and their trafficking on microtubules to the periphery. This work provides a molecular mechanism for KIF1C recruitment to mRNA cargo and reveals a motor-adaptor complex for mRNA transport to cell protrusions.

The role of CNBP in brain atrophy and its targeting in myotonic dystrophy type 2

Myotonic Dystrophy type 2 (DM2) is a multisystem disease affecting many tissues, including skeletal muscle, heart, and brain. DM2 is caused by unstable expansion of CCTG repeats in an intron 1 of a gene coding for cellular nuclear binding protein (CNBP). The expanded CCTG repeats cause DM2 pathology due to the accumulation of RNA CCUG repeats, which affect RNA processing in patients' cells. We have previously shown that mutant CCUG repeats reduce CNBP protein in DM2 patients. Reducing Cnbp in Cnbp KO mouse model causes late skeletal muscle atrophy. In this study, we examined if the reduction of Cnbp affects the Central Nervous System (CNS). MRI and DTI analyses showed that total brain volume and grey matter are reduced in Cnbp KO mice, while mean, radial and axonal brain diffusivity is increased. The morphological changes in the brains of Cnbp KO mice are accompanied by reduced stereotypic behavior, anxiety and neuromotor defects. These findings suggest that the reduction of CNBP contributes to CNS pathology in DM2. Since CNBP stability is regulated by pAMPK-dependent phosphorylation, we examined protein levels of pAMPK in DM2 cells and found that the active pAMPK is reduced in DM2. Interaction of CNBP with pAMPK and stability of CNBP protein are also decreased in DM2. Our data show that a small molecule AMPK activator A769662 corrects CNBP stability and normalizes CNBP targets in DM2 fibroblasts. Thus, activators of AMPK could potentially be developed as therapeutics to correct CNBP and reduce muscle and brain atrophies in DM2.

A KIF1C-CNBP motor-adaptor complex for trafficking mRNAs to cell protrusions

mRNA localization to subcellular compartments is a widely used mechanism that functionally contributes to numerous processes. mRNA targeting can be achieved upon recognition of RNA cargo by molecular motors. However, our molecular understanding of how this is accomplished is limited, especially in higher organisms. We focus on a pathway that targets mRNAs to peripheral protrusions of mammalian cells and is important for cell migration. Trafficking occurs through active transport on microtubules, mediated by the KIF1C kinesin. Here, we identify the RNA-binding protein CNBP, as a factor required for mRNA localization to protrusions. CNBP binds directly to GA-rich sequences in the 3'UTR of protrusion targeted mRNAs. CNBP also interacts with KIF1C and is required for KIF1C recruitment to mRNAs and for their trafficking on microtubules to the periphery. This work provides a molecular mechanism for KIF1C recruitment to mRNA cargo and reveals a motor-adaptor complex for mRNA transport to cell protrusions.

Modeling Myotonic Dystrophy Type 2 Using Drosophila melanogaster

Myotonic dystrophy 2 (DM2) is a genetic multi-systemic disease primarily affecting skeletal muscle. It is caused by CCTGn expansion in intron 1 of the CNBP gene, which encodes a zinc finger protein. DM2 disease has been successfully modeled in Drosophila melanogaster, allowing the identification and validation of new pathogenic mechanisms and potential therapeutic strategies. Here, we describe the principal tools used in Drosophila to study and dissect molecular pathways related to muscular dystrophies and summarize the main findings in DM2 pathogenesis based on DM2 Drosophila models. We also illustrate how Drosophila may be successfully used to generate a tractable animal model to identify novel genes able to affect and/or modify the pathogenic pathway and to discover new potential drugs.

Development of Therapeutic Approaches for Myotonic Dystrophies Type 1 and Type 2

Myotonic Dystrophies type 1 (DM1) and type 2 (DM2) are complex multisystem diseases without disease-based therapies. These disorders are caused by the expansions of unstable CTG (DM1) and CCTG (DM2) repeats outside of the coding regions of the disease genes: DMPK in DM1 and CNBP in DM2. Multiple clinical and molecular studies provided a consensus for DM1 pathogenesis, showing that the molecular pathophysiology of DM1 is associated with the toxicity of RNA CUG repeats, which cause multiple disturbances in RNA metabolism in patients' cells. As a result, splicing, translation, RNA stability and transcription of multiple genes are misregulated in DM1 cells. While mutant CCUG repeats are the main cause of DM2, additional factors might play a role in DM2 pathogenesis. This review describes current progress in the translation of mechanistic knowledge in DM1 and DM2 to clinical trials, with a focus on the development of disease-specific therapies for patients with adult forms of DM1 and congenital DM1 (CDM1).

Translational control of polyamine metabolism by CNBP is required for Drosophila locomotor function

Microsatellite expansions of CCTG repeats in the cellular nucleic acid-binding protein (CNBP) gene leads to accumulation of toxic RNA and have been associated with myotonic dystrophy type 2 (DM2). However, it is still unclear whether the dystrophic phenotype is also linked to CNBP decrease, a conserved CCHC-type zinc finger RNA-binding protein that regulates translation and is required for mammalian development. Here, we show that depletion of Drosophila CNBP in muscles causes ageing-dependent locomotor defects that are correlated with impaired polyamine metabolism. We demonstrate that the levels of ornithine decarboxylase (ODC) and polyamines are significantly reduced upon dCNBP depletion. Of note, we show a reduction of the CNBP-polyamine axis in muscles from DM2 patients. Mechanistically, we provide evidence that dCNBP controls polyamine metabolism through binding dOdc mRNA and regulating its translation. Remarkably, the locomotor defect of dCNBP-deficient flies is rescued by either polyamine supplementation or dOdc1 overexpression. We suggest that this dCNBP function is evolutionarily conserved in vertebrates with relevant implications for CNBP-related pathophysiological conditions.

Action and function of helicases on RNA G-quadruplexes

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Methodological progresses and piling evidence prove the rG4 biology in vivo.

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rG4s step in virtually every aspect of RNA biology.

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Helicases unwinding of rG4s is a fine regulatory layer to the downstream processes and general cell homeostasis.

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The current knowledge is however limited to a few cell lines.

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The regulation of helicases themselves is delineating as a important question.

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Non-helicase rG4-processing proteins likely play a role.

The nucleic acid structure called G-quadruplex (G4) is currently discussed to function in nucleic acid-based mechanisms that influence several cellular processes. They can modulate the cellular machinery either positively or negatively, both at the DNA and RNA level. The majority of what we know about G4 biology comes from DNA G4 (dG4) research. RNA G4s (rG4), on the other hand, are gaining interest as researchers become more aware of their role in several aspects of cellular homeostasis. In either case, the correct regulation of G4 structures within cells is essential and demands specialized proteins able to resolve them. Small changes in the formation and unfolding of G4 structures can have severe consequences for the cells that could even stimulate genome instability, apoptosis or proliferation. Helicases are the most relevant negative G4 regulators, which prevent and unfold G4 formation within cells during different pathways. Yet, and despite their importance only a handful of rG4 unwinding helicases have been identified and characterized thus far. This review addresses the current knowledge on rG4s-processing helicases with a focus on methodological approaches. An example of a non-helicase rG4s-unwinding protein is also briefly described.

What's new about CNBP? Divergent functions and activities for a conserved nucleic acid binding protein

BACKGROUND

Cellular nucleic acid binding protein (CNBP) is a conserved single-stranded nucleic acid binding protein present in most eukaryotes, but not in plants. Expansions in the CNBP gene cause myotonic dystrophy type 2. Initially reported as a transcriptional regulator, CNBP was then also identified acting as a translational regulator.

SCOPE OF REVIEW

The focus of this review was to link the CNBP structural features and newly reported biochemical activities with the recently described biological functions, in the context of its pathological significance.

MAJOR CONCLUSIONS

Several post-translational modifications affect CNBP subcellular localization and activity. CNBP participates in the transcriptional and translational regulation of a wide range of genes by remodeling single-stranded nucleic acid secondary structures and/or by modulating the activity of trans-acting factors. CNBP is required for proper neural crest and heart development, and plays a role in cell proliferation control. Besides, CNBP has been linked with neurodegenerative, inflammatory, and congenital diseases, as well as with tumor processes.

GENERAL SIGNIFICANCE

This review provides an insight into the growing functions of CNBP in cell biology. A unique and robust mechanistic or biochemical connection among these roles has yet not been elucidated. However, the ability of CNBP to dynamically integrate signaling pathways and to act as nucleic acid chaperone may explain most of the roles and functions identified so far.

CDK12 inhibition reduces abnormalities in cells from patients with myotonic dystrophy and in a mouse model

Myotonic dystrophy type 1 (DM1) is an RNA-based disease with no current treatment. It is caused by a transcribed CTG repeat expansion within the 3' untranslated region of the dystrophia myotonica protein kinase (DMPK) gene. Mutant repeat expansion transcripts remain in the nuclei of patients' cells, forming distinct microscopically detectable foci that contribute substantially to the pathophysiology of the condition. Here, we report small-molecule inhibitors that remove nuclear foci and have beneficial effects in the HSA^LR mouse model, reducing transgene expression, leading to improvements in myotonia, splicing, and centralized nuclei. Using chemoproteomics in combination with cell-based assays, we identify cyclin-dependent kinase 12 (CDK12) as a druggable target for this condition. CDK12 is a protein elevated in DM1 cell lines and patient muscle biopsies, and our results showed that its inhibition led to reduced expression of repeat expansion RNA. Some of the inhibitors identified in this study are currently the subject of clinical trials for other indications and provide valuable starting points for a drug development program in DM1.

Cerebral involvement and related aspects in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is an autosomal dominant multisystemic disorder caused by CCTG repeats expansion in the first intron of the CNBP gene. In this review we focus on the brain involvement in DM2, including its pathogenic mechanisms, microstructural, macrostructural and functional brain changes, as well as the effects of all these impairments on patients' everyday life. We also try to understand how brain abnormalities in DM2 should be adequately measured and potentially treated. The most important pathogenetic mechanisms in DM2 are RNA gain-of-function and repeat-associated non-ATG (RAN) translation. One of the main neuroimaging findings in DM2 is the presence of diffuse periventricular white matter hyperintensity lesions (WMHLs). Brain atrophy has been described in DM2 patients, but it is not clear if it is mostly caused by a decrease of the white or gray matter volume. The most commonly reported specific cognitive symptoms in DM2 are dysexecutive syndrome, visuospatial and memory impairments. Fatigue, sleep-related disorders and pain are also frequent in DM2. The majority of key symptoms and signs in DM2 has a great influence on patients' daily lives, their psychological status, economic situation and quality of life.

A deep analysis of the proteomic and phosphoproteomic alterations that occur in skeletal muscle after the onset of immobilization

KEY POINTS

A decrease in protein synthesis plays a major role in the loss of muscle mass that occurs in response to immobilization. In mice, immobilization leads to a rapid (within 6 h) and progressive decrease in the rate of protein synthesis and this effect is mediated by a decrease in translational efficiency. Deep proteomic and phosphoproteomic analyses of mouse skeletal muscles revealed that the rapid immobilization-induced decrease in protein synthesis cannot be explained by changes in the abundance or phosphorylation state of proteins that have been implicated in the regulation of translation.

ABSTRACT

The disuse of skeletal muscle, such as that which occurs during immobilization, can lead to the rapid loss of muscle mass, and a decrease in the rate of protein synthesis plays a major role in this process. Indeed, current dogma contends that the decrease in protein synthesis is mediated by changes in the activity of protein kinases (e.g. mTOR); however, the validity of this model has not been established. Therefore, to address this, we first subjected mice to 6, 24 or 72 h of unilateral immobilization and then used the SUnSET technique to measure changes in the relative rate of protein synthesis. The result of our initial experiments revealed that immobilization leads to a rapid (within 6 h) and progressive decrease in the rate of protein synthesis and that this effect is mediated by a decrease in translational efficiency. We then performed a deep mass spectrometry-based analysis to determine whether this effect could be explained by changes in the expression and/or phosphorylation state of proteins that regulate translation. From this analysis, we were able to quantify 4320 proteins and 15,020 unique phosphorylation sites, and surprisingly, the outcomes revealed that the rapid immobilization-induced decrease in protein synthesis could not be explained by changes in either the abundance, or phosphorylation state, of proteins. The results of our work not only challenge the current dogma in the field, but also provide an expansive resource of information for future studies that are aimed at defining how disuse leads to loss of muscle mass.

The distinct roles of zinc finger CCHC-type (ZCCHC) superfamily proteins in the regulation of RNA metabolism

The zinc finger CCHC-type (ZCCHC) superfamily proteins, characterized with the consensus sequence C-X₂-C-X₄-H-X₄-C, are accepted to have high-affinity binding to single-stranded nucleic acids, especially single-stranded RNAs. In human beings 25 ZCCHC proteins have been annotated in the HGNC database. Of interest is that among the family, most members are involved in the multiple steps of RNA metabolism. In this review, we focus on the diverged roles of human ZCCHC proteins on RNA transcription, biogenesis, splicing, as well as translation and degradation.

Control of ribosomal protein synthesis by the Microprocessor complex

Ribosome biogenesis in eukaryotes requires the coordinated production and assembly of 80 ribosomal proteins and four ribosomal RNAs (rRNAs), and its rate must be synchronized with cellular growth. Here, we showed that the Microprocessor complex, which mediates the first step of microRNA processing, potentiated the transcription of ribosomal protein genes by eliminating DNA/RNA hybrids known as R-loops. Nutrient deprivation triggered the nuclear export of Drosha, a key component of the Microprocessor complex, and its subsequent degradation by the E3 ubiquitin ligase Nedd4, thereby reducing ribosomal protein production and protein synthesis. In mouse erythroid progenitors, conditional deletion of Drosha led to the reduced production of ribosomal proteins, translational inhibition of the mRNA encoding the erythroid transcription factor Gata1, and impaired erythropoiesis. This phenotype mirrored the clinical presentation of human "ribosomopathies." Thus, the Microprocessor complex plays a pivotal role in synchronizing protein synthesis capacity with cellular growth rate and is a potential drug target for anemias caused by ribosomal insufficiency.

Myotonic dystrophy type 2: the 2020 update

The myotonic dystrophies are the commonest cause of adult-onset muscular dystrophy. Phenotypes of DM1 and DM2 are similar, but there are some important differences, including the presence or absence of congenital form, muscles primarily affected (distal vs proximal), involved muscle fiber types (type 1 vs type 2 fibers), and some associated multisystemic phenotypes. There is currently no cure for the myotonic dystrophies but effective management significantly reduces the morbidity and mortality of patients. For the enormous understanding of the molecular pathogenesis of myotonic dystrophy type 1 and myotonic dystrophy type 2, these diseases are now called "spliceopathies" and are mediated by a primary disorder of RNA rather than proteins. Despite clinical and genetic similarities, myotonic dystrophy type 1 and type 2 are distinct disorders requiring different diagnostic and management strategies. Gene therapy for myotonic dystrophy type 1 and myotonic dystrophy type 2 appears to be very close and the near future is an exciting time for clinicians and patients.

Antisense Transcription across Nucleotide Repeat Expansions in Neurodegenerative and Neuromuscular Diseases: Progress and Mysteries

Unstable repeat expansions and insertions cause more than 30 neurodegenerative and neuromuscular diseases. Remarkably, bidirectional transcription of repeat expansions has been identified in at least 14 of these diseases. More remarkably, a growing number of studies has been showing that both sense and antisense repeat RNAs are able to dysregulate important cellular pathways, contributing together to the observed clinical phenotype. Notably, antisense repeat RNAs from spinocerebellar ataxia type 7, myotonic dystrophy type 1, Huntington's disease and frontotemporal dementia/amyotrophic lateral sclerosis associated genes have been implicated in transcriptional regulation of sense gene expression, acting either at a transcriptional or posttranscriptional level. The recent evidence that antisense repeat RNAs could modulate gene expression broadens our understanding of the pathogenic pathways and adds more complexity to the development of therapeutic strategies for these disorders. In this review, we cover the amazing progress made in the understanding of the pathogenic mechanisms associated with repeat expansion neurodegenerative and neuromuscular diseases with a focus on the impact of antisense repeat transcription in the development of efficient therapies.

Genome-wide analysis of CCHC-type zinc finger (ZCCHC) proteins in yeast, Arabidopsis, and humans

The diverse eukaryotic proteins that contain zinc fingers participate in many aspects of nucleic acid metabolism, from DNA transcription to RNA degradation, post-transcriptional gene silencing, and small RNA biogenesis. These proteins can be classified into at least 30 types based on structure. In this review, we focus on the CCHC-type zinc fingers (ZCCHC), which contain an 18-residue domain with the CX2CX4HX4C sequence, where C is cysteine, H is histidine, and X is any amino acid. This motif, also named the "zinc knuckle", is characteristic of the retroviral Group Antigen protein and occurs alone or with other motifs. Many proteins containing zinc knuckles have been identified in eukaryotes, but only a few have been studied. Here, we review the available information on ZCCHC-containing factors from three evolutionarily distant eukaryotes-Saccharomyces cerevisiae, Arabidopsis thaliana, and Homo sapiens-representing fungi, plants, and metazoans, respectively. We performed systematic searches for proteins containing the CX2CX4HX4C sequence in organism-specific and generalist databases. Next, we analyzed the structural and functional information for all such proteins stored in UniProtKB. Excluding retrotransposon-encoded proteins and proteins harboring uncertain ZCCHC motifs, we found seven ZCCHC-containing proteins in yeast, 69 in Arabidopsis, and 34 in humans. ZCCHC-containing proteins mainly localize to the nucleus, but some are nuclear and cytoplasmic, or exclusively cytoplasmic, and one localizes to the chloroplast. Most of these factors participate in RNA metabolism, including transcriptional elongation, polyadenylation, translation, pre-messenger RNA splicing, RNA export, RNA degradation, microRNA and ribosomal RNA biogenesis, and post-transcriptional gene silencing. Several human ZCCHC-containing factors are derived from neofunctionalized retrotransposons and act as proto-oncogenes in diverse neoplastic processes. The conservation of ZCCHCs in orthologs of these three phylogenetically distant eukaryotes suggests that these domains have biologically relevant functions that are not well known at present.

TOP mRNPs: Molecular Mechanisms and Principles of Regulation

The cellular response to changes in the surrounding environment and to stress requires the coregulation of gene networks aiming to conserve energy and resources. This is often achieved by downregulating protein synthesis. The 5’ Terminal OligoPyrimidine (5’ TOP) motif-containing mRNAs, which encode proteins that are essential for protein synthesis, are the primary targets of translational control under stress. The TOP motif is a cis-regulatory RNA element that begins directly after the m7G cap structure and contains the hallmark invariant 5’-cytidine followed by an uninterrupted tract of 4–15 pyrimidines. Regulation of translation via the TOP motif coordinates global protein synthesis with simultaneous co-expression of the protein components required for ribosome biogenesis. In this review, we discuss architecture of TOP mRNA-containing ribonucleoprotein complexes, the principles of their assembly, and the modes of regulation of TOP mRNA translation.

AON-induced splice-switching and DMPK pre-mRNA degradation as potential therapeutic approaches for Myotonic Dystrophy type 1

Expansion of an unstable CTG repeat in the 3′UTR of the DMPK gene causes Myotonic Dystrophy type 1 (DM1). CUG-expanded DMPK transcripts (CUG^exp) sequester Muscleblind-like (MBNL) alternative splicing regulators in ribonuclear inclusions (foci), leading to abnormalities in RNA processing and splicing. To alleviate the burden of CUG^exp, we tested therapeutic approach utilizing antisense oligonucleotides (AONs)-mediated DMPK splice-switching and degradation of mutated pre-mRNA. Experimental design involved: (i) skipping of selected constitutive exons to induce frameshifting and decay of toxic mRNAs by an RNA surveillance mechanism, and (ii) exclusion of the alternative exon 15 (e15) carrying CUG^exp from DMPK mRNA. While first strategy failed to stimulate DMPK mRNA decay, exclusion of e15 enhanced DMPK nuclear export but triggered accumulation of potentially harmful spliced out pre-mRNA fragment containing CUG^exp. Neutralization of this fragment with antisense gapmers complementary to intronic sequences preceding e15 failed to diminish DM1-specific spliceopathy due to AONs’ chemistry-related toxicity. However, intronic gapmers alone reduced the level of DMPK mRNA and mitigated DM1-related cellular phenotypes including spliceopathy and nuclear foci. Thus, a combination of the correct chemistry and experimental approach should be carefully considered to design a safe AON-based therapeutic strategy for DM1.

Short Tandem Repeat Expansions and RNA-Mediated Pathogenesis in Myotonic Dystrophy

Short tandem repeat (STR) or microsatellite, expansions underlie more than 50 hereditary neurological, neuromuscular and other diseases, including myotonic dystrophy types 1 (DM1) and 2 (DM2). Current disease models for DM1 and DM2 propose a common pathomechanism, whereby the transcription of mutant DMPK (DM1) and CNBP (DM2) genes results in the synthesis of CUG and CCUG repeat expansion (CUG^exp, CCUG^exp) RNAs, respectively. These CUG^exp and CCUG^exp RNAs are toxic since they promote the assembly of ribonucleoprotein (RNP) complexes or RNA foci, leading to sequestration of Muscleblind-like (MBNL) proteins in the nucleus and global dysregulation of the processing, localization and stability of MBNL target RNAs. STR expansion RNAs also form phase-separated gel-like droplets both in vitro and in transiently transfected cells, implicating RNA-RNA multivalent interactions as drivers of RNA foci formation. Importantly, the nucleation and growth of these nuclear foci and transcript misprocessing are reversible processes and thus amenable to therapeutic intervention. In this review, we provide an overview of potential DM1 and DM2 pathomechanisms, followed by a discussion of MBNL functions in RNA processing and how multivalent interactions between expanded STR RNAs and RNA-binding proteins (RBPs) promote RNA foci assembly.

Myoediting: Toward Prevention of Muscular Dystrophy by Therapeutic Genome Editing

Muscular dystrophies represent a large group of genetic disorders that significantly impair quality of life and often progress to premature death. There is no effective treatment for these debilitating diseases. Most therapies, developed to date, focus on alleviating the symptoms or targeting the secondary effects, while the underlying gene mutation is still present in the human genome. The discovery and application of programmable nucleases for site-specific DNA double-stranded breaks provides a powerful tool for precise genome engineering. In particular, the CRISPR/Cas system has revolutionized the genome editing field and is providing a new path for disease treatment by targeting the disease-causing genetic mutations. In this review, we provide a historical overview of genome-editing technologies, summarize the most recent advances, and discuss potential strategies and challenges for permanently correcting genetic mutations that cause muscular dystrophies.

Reduction of Cellular Nucleic Acid Binding Protein Encoded by a Myotonic Dystrophy Type 2 Gene Causes Muscle Atrophy

Myotonic dystrophy type 2 (DM2) is a neuromuscular disease caused by an expansion of intronic CCTG repeats in the CNBP gene, which encodes a protein regulating translation and transcription. To better understand the role of cellular nucleic acid binding protein (CNBP) in DM2 pathology, we examined skeletal muscle in a new model of Cnbp knockout (KO) mice. This study showed that a loss of Cnbp disturbs myofibrillar sarcomeric organization at birth. Surviving homozygous Cnbp KO mice develop muscle atrophy at a young age. The skeletal muscle phenotype in heterozygous Cnbp KO mice was milder, but they developed severe muscle wasting at an advanced age. Several proteins that control global translation and muscle contraction are altered in muscle of Cnbp KO mice. A search for CNBP binding proteins showed that CNBP interacts with the α subunit of the dystroglycan complex, a core component of the multimeric dystrophin-glycoprotein complex, which regulates membrane stability. Whereas CNBP is reduced in cytoplasm of DM2 human fibers, it is a predominantly membrane protein in DM2 fibers, and its interaction with α-dystroglycan is increased in DM2. These findings suggest that alterations of CNBP in DM2 might cause muscle atrophy via CNBP-mediated translation and via protein-protein interactions affecting myofiber membrane function.

Abnormalities in Skeletal Muscle Myogenesis, Growth, and Regeneration in Myotonic Dystrophy

Myotonic dystrophy type 1 (DM1) and 2 (DM2) are autosomal dominant degenerative neuromuscular disorders characterized by progressive skeletal muscle weakness, atrophy, and myotonia with progeroid features. Although both DM1 and DM2 are characterized by skeletal muscle dysfunction and also share other clinical features, the diseases differ in the muscle groups that are affected. In DM1, distal muscles are mainly affected, whereas in DM2 problems are mostly found in proximal muscles. In addition, manifestation in DM1 is generally more severe, with possible congenital or childhood-onset of disease and prominent CNS involvement. DM1 and DM2 are caused by expansion of (CTG•CAG)n and (CCTG•CAGG)n repeats in the 3' non-coding region of DMPK and in intron 1 of CNBP, respectively, and in overlapping antisense genes. This critical review will focus on the pleiotropic problems that occur during development, growth, regeneration, and aging of skeletal muscle in patients who inherited these expansions. The current best-accepted idea is that most muscle symptoms can be explained by pathomechanistic effects of repeat expansion on RNA-mediated pathways. However, aberrations in DNA replication and transcription of the DM loci or in protein translation and proteome homeostasis could also affect the control of proliferation and differentiation of muscle progenitor cells or the maintenance and physiological integrity of muscle fibers during a patient's lifetime. Here, we will discuss these molecular and cellular processes and summarize current knowledge about the role of embryonic and adult muscle-resident stem cells in growth, homeostasis, regeneration, and premature aging of healthy and diseased muscle tissue. Of particular interest is that also progenitor cells from extramuscular sources, such as pericytes and mesoangioblasts, can participate in myogenic differentiation. We will examine the potential of all these types of cells in the application of regenerative medicine for muscular dystrophies and evaluate new possibilities for their use in future therapy of DM.

Intron retention induced by microsatellite expansions as a disease biomarker

Expansions of simple sequence repeats, or microsatellites, have been linked to ∼30 neurological-neuromuscular diseases. While these expansions occur in coding and noncoding regions, microsatellite sequence and repeat length diversity is more prominent in introns with eight different trinucleotide to hexanucleotide repeats, causing hereditary diseases such as myotonic dystrophy type 2 (DM2), Fuchs endothelial corneal dystrophy (FECD), and C9orf72 amyotrophic lateral sclerosis and frontotemporal dementia (C9-ALS/FTD). Here, we test the hypothesis that these GC-rich intronic microsatellite expansions selectively trigger host intron retention (IR). Using DM2, FECD, and C9-ALS/FTD as examples, we demonstrate that retention is readily detectable in affected tissues and peripheral blood lymphocytes and conclude that IR screening constitutes a rapid and inexpensive biomarker for intronic repeat expansion 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.

The Human CCHC-type Zinc Finger Nucleic Acid-Binding Protein Binds G-Rich Elements in Target mRNA Coding Sequences and Promotes Translation

The CCHC-type zinc finger nucleic acid-binding protein (CNBP/ZNF9) is conserved in eukaryotes and is essential for embryonic development in mammals. It has been implicated in transcriptional, as well as post-transcriptional, gene regulation; however, its nucleic acid ligands and molecular function remain elusive. Here, we use multiple systems-wide approaches to identify CNBP targets and function. We used photoactivatable ribonucleoside-enhanced crosslinking and immunoprecipitation (PAR-CLIP) to identify 8,420 CNBP binding sites on 4,178 mRNAs. CNBP preferentially bound G-rich elements in the target mRNA coding sequences, most of which were previously found to form G-quadruplex and other stable structures in vitro. Functional analyses, including RNA sequencing, ribosome profiling, and quantitative mass spectrometry, revealed that CNBP binding did not influence target mRNA abundance but rather increased their translational efficiency. Considering that CNBP binding prevented G-quadruplex structure formation in vitro, we hypothesize that CNBP is supporting translation by resolving stable structures on mRNAs.

Expanded CCUG repeat RNA expression in Drosophila heart and muscle trigger Myotonic Dystrophy type 1-like phenotypes and activate autophagocytosis genes

Myotonic dystrophies (DM1–2) are neuromuscular genetic disorders caused by the pathological expansion of untranslated microsatellites. DM1 and DM2, are caused by expanded CTG repeats in the 3′UTR of the DMPK gene and CCTG repeats in the first intron of the CNBP gene, respectively. Mutant RNAs containing expanded repeats are retained in the cell nucleus, where they sequester nuclear factors and cause alterations in RNA metabolism. However, for unknown reasons, DM1 is more severe than DM2. To study the differences and similarities in the pathogenesis of DM1 and DM2, we generated model flies by expressing pure expanded CUG ([250]×) or CCUG ([1100]×) repeats, respectively, and compared them with control flies expressing either 20 repeat units or GFP. We observed surprisingly severe muscle reduction and cardiac dysfunction in CCUG-expressing model flies. The muscle and cardiac tissue of both DM1 and DM2 model flies showed DM1-like phenotypes including overexpression of autophagy-related genes, RNA mis-splicing and repeat RNA aggregation in ribonuclear foci along with the Muscleblind protein. These data reveal, for the first time, that expanded non-coding CCUG repeat-RNA has similar in vivo toxicity potential as expanded CUG RNA in muscle and heart tissues and suggests that specific, as yet unknown factors, quench CCUG-repeat toxicity in DM2 patients.

Biomolecular diagnosis of myotonic dystrophy type 2: a challenging approach

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are the most common adult form of muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia, and multiorgan involvement. The onset and symptoms of the myotonic dystrophies are diverse, complicating their diagnoses and limiting a comprehensive approach to their clinical care. Diagnostic delay in DM2 is due not only to the heterogeneous phenotype and the aspecific onset but also to the unfamiliarity with the disorder by most clinicians. Moreover, the DM2 diagnostic odyssey is complicated by the difficulties to develop an accurate, robust, and cost-effective method for a routine molecular assay. The aim of this review is to underline by challenging approach the diagnostic limits and pitfalls that could results in failure to recognize the presence of DM2 disease. Understanding and preventing delays in DM2 diagnosis may facilitate family planning, improve symptom management in the short term, and facilitate more specific treatment in the long term.

Myotonic dystrophy type 2 and modifier genes: an update on clinical and pathomolecular aspects

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia, and multiorgan involvement. To date, two distinct forms caused by similar mutations in two different genes have been identified: myotonic dystrophy type 1 (DM1) and myotonic dystrophy type 2 (DM2). Aberrant transcription and mRNA processing of multiple genes due to RNA-mediated toxic gain-of function has been suggested to cause the complex phenotype in DM1 and DM2. However, despite clinical and genetic similarities, DM1 and DM2 may be considered as distinct disorders. This review is an update on the latest findings specific to DM2, including explanations for the differences in clinical manifestations and pathophysiology between the two forms of myotonic dystrophies.

An Inhibitory Motif on the 5'UTR of Several Rotavirus Genome Segments Affects Protein Expression and Reverse Genetics Strategies

Rotavirus genome consists of eleven segments of dsRNA, each encoding one single protein. Viral mRNAs contain an open reading frame (ORF) flanked by relatively short untranslated regions (UTRs), whose role in the viral cycle remains elusive. Here we investigated the role of 5'UTRs in T7 polymerase-driven cDNAs expression in uninfected cells. The 5'UTRs of eight genome segments (gs3, gs5-6, gs7-11) of the simian SA11 strain showed a strong inhibitory effect on the expression of viral proteins. Decreased protein expression was due to both compromised transcription and translation and was independent of the ORF and the 3'UTR sequences. Analysis of several mutants of the 21-nucleotide long 5'UTR of gs 11 defined an inhibitory motif (IM) represented by its primary sequence rather than its secondary structure. IM was mapped to the 5' terminal 6-nucleotide long pyrimidine-rich tract 5'-GGY(U/A)UY-3'. The 5' terminal position within the mRNA was shown to be essentially required, as inhibitory activity was lost when IM was moved to an internal position. We identified two mutations (insertion of a G upstream the 5'UTR and the U to A mutation of the fifth nucleotide of IM) that render IM non-functional and increase the transcription and translation rate to levels that could considerably improve the efficiency of virus helper-free reverse genetics strategies.

Bridging integrator 1 (BIN1) protein expression increases in the Alzheimer's disease brain and correlates with neurofibrillary tangle pathology

Recent genome wide association studies have implicated bridging integrator 1 (BIN1) as a late-onset Alzheimer's disease (AD) susceptibility gene. There are at least 15 different known isoforms of BIN1, with many being expressed in the brain including the longest isoform (iso1), which is brain-specific and localizes to axon initial segments and nodes of Ranvier. It is currently unknown what role BIN1 plays in AD. We analyzed BIN1 protein expression from a large number (n = 71) of AD cases and controls from five different brain regions (hippocampus, inferior parietal cortex, inferior temporal cortex, frontal cortex (BA9), and superior and middle temporal gyri). We found that the amount of the largest isoform of BIN1 was significantly reduced in the AD brain compared to age-matched controls, and smaller BIN1 isoforms were significantly increased. Further, BIN1 was significantly correlated with the amount of neurofibrillary tangle (NFT) pathology but not with either diffuse or neuritic plaques, or with the amount of amyloid-β peptide. BIN1 is known to be abnormally expressed in another human disease, myotonic dystrophy, which also features prominent NFT pathology. These data suggest that BIN1 is likely involved in AD as a modulator of NFT pathology, and that this role may extend to other human diseases that feature tau pathology.

Myotonic Dystrophy Type 2: An Update on Clinical Aspects, Genetic and Pathomolecular Mechanism

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in CNBP. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders. The pathogenesis of DM is explained by a common RNA gain-of-function mechanism in which the CUG and CCUG repeats alter cellular function, including alternative splicing of various genes. However additional pathogenic mechanism like changes in gene expression, modifier genes, protein translation and micro-RNA metabolism may also contribute to disease pathology and to clarify the phenotypic differences between these two types of myotonic dystrophies.This review is an update on the latest findings specific to DM2, including explanations for the differences in clinical manifestations and pathophysiology between the two forms of myotonic dystrophies.

Poly(A)-binding proteins: structure, domain organization, and activity regulation

RNA-binding proteins are of vital importance for mRNA functioning. Among these, poly(A)-binding proteins (PABPs) are of special interest due to their participation in virtually all mRNA-dependent events that is caused by their high affinity for A-rich mRNA sequences. Apart from mRNAs, PABPs interact with many proteins, thus promoting their involvement in cellular events. In the nucleus, PABPs play a role in polyadenylation, determine the length of the poly(A) tail, and may be involved in mRNA export. In the cytoplasm, they participate in regulation of translation initiation and either protect mRNAs from decay through binding to their poly(A) tails or stimulate this decay by promoting mRNA interactions with deadenylase complex proteins. This review presents modern notions of the role of PABPs in mRNA-dependent events; peculiarities of regulation of PABP amount in the cell and activities are also discussed.

The race to decipher the top secrets of TOP mRNAs

Cells encountering hostile growth conditions, like those residing in the middle of a newly developing solid tumor, conserve resources and energy by downregulating protein synthesis. One mechanism in this response is the translational repression of multiple mRNAs that encode components of the translational apparatus. This coordinated translational control is carried through a common cis-regulatory element, the 5' Terminal OligoPyrimidine motif (5'TOP), after which these mRNAs are referred to as TOP mRNAs. Subsequent to the initial structural and functional characterization of members of this family, the research of TOP mRNAs has progressed in three major directions: a) delineating the landscape of the family; b) establishing the pathways that transduce stress cues into selective translational repression; and c) attempting to decipher the most proximal trans-acting factor(s) and defining its mode of action--a repressor or activator. The present chapter critically reviews the development in these three avenues of research with a special emphasis on the two "top secrets" of the TOP mRNA family: the scope of its members and the identity of the proximal cellular regulator(s). This article is part of a Special Issue entitled: Translation and Cancer.

RAN translation and frameshifting as translational challenges at simple repeats of human neurodegenerative disorders

Repeat-associated disorders caused by expansions of short sequences have been classified as coding and noncoding and are thought to be caused by protein gain-of-function and RNA gain-of-function mechanisms, respectively. The boundary between such classifications has recently been blurred by the discovery of repeat-associated non-AUG (RAN) translation reported in spinocerebellar ataxia type 8, myotonic dystrophy type 1, fragile X tremor/ataxia syndrome and C9ORF72 amyotrophic lateral sclerosis and frontotemporal dementia. This noncanonical translation requires no AUG start codon and can initiate in multiple frames of CAG, CGG and GGGGCC repeats of the sense and antisense strands of disease-relevant transcripts. RNA structures formed by the repeats have been suggested as possible triggers; however, the precise mechanism of the translation initiation remains elusive. Templates containing expansions of microsatellites have also been shown to challenge translation elongation, as frameshifting has been recognized across CAG repeats in spinocerebellar ataxia type 3 and Huntington's disease. Determining the critical requirements for RAN translation and frameshifting is essential to decipher the mechanisms that govern these processes. The contribution of unusual translation products to pathogenesis needs to be better understood. In this review, we present current knowledge regarding RAN translation and frameshifting and discuss the proposed mechanisms of translational challenges imposed by simple repeat expansions.

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.

Progression of muscle histopathology but not of spliceopathy in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is an autosomal dominant progressive disease involving skeletal and cardiac muscle and brain. It is caused by a tetranucleotide repeat within the first intron of the CNBP gene that leads to an alteration of the alternative splicing of several genes. To understand the molecular mechanisms that play a role in DM2 progression, the evolution of skeletal muscle histopathology and biomolecular findings in successive biopsies have been studied. Biceps brachii biopsies from 5 DM2 patients who underwent two successive biopsies at different years of age have been used. Muscle histopathology has been assessed on sections immunostained with fast or slow myosin. FISH in combination with MBNL1-immunofluorescence has been performed to evaluate ribonuclear inclusion and MBNL1 foci dimensions in myonuclei. Gene and protein expression and alteration of alternative splicing of several genes have been evaluated over time. All DM2 patients examined show a worsening of muscle histopathology and an increase of foci dimensions over time. The progressive worsening of myotonia in DM2 patients may be due to the decrease of CLCN1 mRNA observed in all patients examined. However, a worsening of alternative splicing alterations has not been evidenced over time. The data obtained in this study confirm that DM2 is a slow progression disease since histological and biomolecular alterations observed in skeletal muscle are minimal even after 10-year interval. The data indicate that muscle morphological alterations evolve more rapidly over time than the molecular changes thus indicating that muscle biopsy is a more sensitive tool than biomolecular markers to assess disease progression at muscle level.

Small molecules that target the toxic RNA in myotonic dystrophy type 2

Myotonic dystrophy type 2 (DM2) is caused by an expansion of CCTG repeats in the zinc-finger protein gene (ZNF9). Transcribed CCUG repeats sequester muscleblind-like protein 1 (MBNL1), an important alternative splicing regulator, preventing its normal function, leading to the disease phenotype. We describe a series of ligands that disrupt the MBNL1-r(CCUG)n interaction as potential lead agents for developing DM2 therapeutics. A previously reported triaminopyrimidine-acridine conjugate was a moderate inhibitor in vitro, however it proved to be poorly water-soluble and not cell-permeable. To improve its therapeutic potential, the new set of ligands maintained the key triaminopyrimidine recognition unit but replaced the acridine intercalator with a bisamidinium groove binder. The optimized ligands exhibit low micromolar inhibition potency to MBNL1-r(CCUG)8. Importantly, the ligands are the first to show the ability to disrupt the MBNL1-r(CCUG)n foci in DM2 model cell culture and exhibit low cytotoxicity.

Myotonic dystrophies: An update on clinical aspects, genetic, pathology, and molecular pathomechanisms

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. However, it is now clear that additional pathogenic mechanism like changes in gene expression, protein translation and micro-RNA metabolism may also contribute to disease pathology. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. This review is an update on the recent advances in the understanding of the molecular mechanisms behind myotonic dystrophies. This article is part of a Special Issue entitled: Neuromuscular Diseases: Pathology and Molecular Pathogenesis.

RNA-binding protein misregulation in microsatellite expansion disorders

RNA-binding proteins (RBPs) play pivotal roles in multiple cellular pathways from transcription to RNA turnover by interacting with RNA sequence and/or structural elements to form distinct RNA-protein complexes. Since these complexes are required for the normal regulation of gene expression, mutations that alter RBP functions may result in a cascade of deleterious events that lead to severe disease. Here, we focus on a group of hereditary disorders, the microsatellite expansion diseases, which alter RBP activities and result in abnormal neurological and neuromuscular phenotypes. While many of these diseases are classified as adult-onset disorders, mounting evidence indicates that disruption of normal RNA-protein interaction networks during embryogenesis modifies developmental pathways, which ultimately leads to disease manifestations later in life. Efforts to understand the molecular basis of these disorders has already uncovered novel pathogenic mechanisms, including RNA toxicity and repeat-associated non-ATG (RAN) translation, and current studies suggest that additional surprising insights into cellular regulatory pathways will emerge in the future.

Overexpression of CUGBP1 in skeletal muscle from adult classic myotonic dystrophy type 1 but not from myotonic dystrophy type 2

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are progressive multisystemic disorders caused by similar mutations at two different genetic loci. The common key feature of DM pathogenesis is nuclear accumulation of mutant RNA which causes aberrant alternative splicing of specific pre-mRNAs by altering the functions of two RNA binding proteins, MBNL1 and CUGBP1. However, DM1 and DM2 show disease-specific features that make them clearly separate diseases suggesting that other cellular and molecular pathways may be involved. In this study we have analysed the histopathological, and biomolecular features of skeletal muscle biopsies from DM1 and DM2 patients in relation to presenting phenotypes to better define the molecular pathogenesis. Particularly, the expression of CUGBP1 protein has been examined to clarify if this factor may act as modifier of disease-specific manifestations in DM. The results indicate that the splicing and muscle pathological alterations observed are related to the clinical phenotype both in DM1 and in DM2 and that CUGBP1 seems to play a role in classic DM1 but not in DM2. In conclusion, our results indicate that multisystemic disease spectrum of DM pathologies may not be explained only by spliceopathy thus confirming that the molecular pathomechanism of DM is more complex than that actually suggested.

Clinical aspects, molecular pathomechanisms and management of myotonic dystrophies

Myotonic dystrophy (DM) is the most common adult muscular dystrophy, characterized by autosomal dominant progressive myopathy, myotonia and multiorgan involvement. To date two distinct forms caused by similar mutations have been identified. Myotonic dystrophy type 1 (DM1, Steinert's disease) was described more than 100 years ago and is caused by a (CTG)n expansion in DMPK, while myotonic dystrophy type 2 (DM2) was identified only 18 years ago and is caused by a (CCTG)n expansion in ZNF9/CNBP. When transcribed into CUG/CCUG-containing RNA, mutant transcripts aggregate as nuclear foci that sequester RNA-binding proteins, resulting in spliceopathy of downstream effector genes. Despite clinical and genetic similarities, DM1 and DM2 are distinct disorders requiring different diagnostic and management strategies. DM1 may present in four different forms: congenital, early childhood, adult onset and late-onset oligosymptomatic DM1. Congenital DM1 is the most severe form of DM characterized by extreme muscle weakness and mental retardation. In DM2 the clinical phenotype is extremely variable and there are no distinct clinical subgroups. Congenital and childhood-onset forms are not present in DM2 and, in contrast to DM1, myotonia may be absent even on EMG. Due to the lack of awareness of the disease among clinicians, DM2 remains largely underdiagnosed. The delay in receiving the correct diagnosis after onset of first symptoms is very long in DM: on average more than 5 years for DM1 and more than 14 years for DM2 patients. The long delay in the diagnosis of DM causes unnecessary problems for the patients to manage their lives and anguish with uncertainty of prognosis and treatment.

CNBP regulates wing development in Drosophila melanogaster by promoting IRES-dependent translation of dMyc

CCHC-type zinc finger nucleic acid binding protein (CNBP) is a small conserved protein, which plays a key role in development and disease. Studies in animal models have shown that the absence of CNBP results in severe developmental defects that have been mostly attributed to its ability to regulate c-myc mRNA expression. Functionally, CNBP binds single-stranded nucleic acids and acts as a molecular chaperone, thus regulating both transcription and translation.   In this work we report that in Drosophila melanogaster, CNBP is an essential gene, whose absence causes early embryonic lethality. In contrast to what observed in other species, ablation of CNBP does not affect dMyc mRNA expression, whereas the protein levels are markedly reduced. We demonstrate for the first time that dCNBP regulates dMyc translation through an IRES-dependent mechanism, and that knockdown of dCNBP in the wing territory causes a general reduction of wing size, in keeping with the reported role of dMyc in this region. Consistently, reintroduction of dMyc in CNBP-deficient wing imaginal discs rescues the wing size, further supporting a key role of the CNBP-Myc axis in this context. Collectively, these data show a previously uncharacterized mechanism, whereby, by regulating dMyc IRES-dependent translation, CNBP controls Drosophila wing development. These results may have relevant implications in other species and in pathophysiological conditions.

Characterization and regulation of mechanical loading-induced compensatory muscle hypertrophy

In mammalian systems, skeletal muscle exists in a dynamic state that monitors and regulates the physiological investment in muscle size to meet the current level of functional demand. This review attempts to consolidate current knowledge concerning development of the compensatory hypertrophy that occurs in response to a sustained increase in the mechanical loading of skeletal muscle. Topics covered include: defining and measuring compensatory hypertrophy, experimental models, loading stimulus parameters, acute responses to increased loading, hyperplasia, myofiber-type adaptations, the involvement of satellite cells, mRNA translational control, mechanotransduction, and endocrinology. The authors conclude with their impressions of current knowledge gaps in the field that are ripe for future study.

Altered expression and splicing of Ca(2+) metabolism genes in myotonic dystrophies DM1 and DM2

AIMS

Myotonic dystrophy types 1 and 2 (DM1 and DM2) are multisystem disorders caused by similar repeat expansion mutations, with similar yet distinct clinical features. Aberrant splicing of multiple effector genes, as well as dysregulation of transcription and translation, has been suggested to underlie different aspects of the complex phenotypes in DM1 and DM2. Ca(2+) plays a central role in both muscle contraction and control of gene expression, and recent expression profiling studies have indicated major perturbations of the Ca(2+) signalling pathways in DM. Here we have further investigated the expression of genes and proteins involved in Ca(2+) metabolism in DM patients, including Ca(2+) channels and Ca(2+) binding proteins.

METHODS

We used patient muscle biopsies to analyse mRNA expression and splicing of genes by microarray expression profiling and RT-PCR. We studied protein expression by immunohistochemistry and immunoblotting.

RESULTS

Most of the genes studied showed mRNA up-regulation in expression profiling. When analysed by immunohistochemistry the Ca(2+) release channel ryanodine receptor was reduced in DM1 and DM2, as was calsequestrin 2, a sarcoplasmic reticulum lumen Ca(2+) storage protein. Abnormal splicing of ATP2A1 was more pronounced in DM2 than DM1.

CONCLUSIONS

We observed abnormal mRNA and protein expression in DM affecting several proteins involved in Ca(2+) metabolism, with some differences between DM1 and DM2. Our protein expression studies are suggestive of a post-transcriptional defect(s) in the myotonic dystrophies.

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.

Novel high-performance purification protocol of recombinant CNBP suitable for biochemical and biophysical characterization

Cellular nucleic acid binding protein (CNBP) is a highly conserved multi-zinc knuckle protein that enhances c-MYC expression, is related to certain human muscular diseases and is required for proper rostral head development. CNBP binds to single-stranded DNA (ssDNA) and RNA and acts as nucleic acid chaperone. Despite the advances made concerning CNBP biological roles, a full knowledge about the structure-function relationship has not yet been achieved, likely due to difficulty in obtaining pure and tag-free CNBP. Here, we report a fast, simple, reproducible, and high-performance expression and purification protocol that provides recombinant tag-free CNBP from Escherichia coli cultures. We determined that tag-free CNBP binds its molecular targets with higher affinity than tagged-CNBP. Furthermore, fluorescence spectroscopy revealed the presence of a unique and conserved tryptophan, which is exposed to the solvent and involved, directly or indirectly, in nucleic acid binding. Size-exclusion HPLC revealed that CNBP forms homodimers independently of nucleic acid binding and coexist with monomers as non-interconvertible forms or in slow equilibrium. Circular dichroism spectroscopy showed that CNBP has a secondary structure dominated by random-coil and β-sheet coincident with the sequence-predicted repetitive zinc knuckles motifs, which folding is required for CNBP structural stability and biochemical activity. CNBP structural stability increased in the presence of single-stranded nucleic acid targets similar to other unstructured nucleic acid chaperones. Altogether, data suggest that CNBP is a flexible protein with interspersed structured zinc knuckles, and acquires a more rigid structure upon nucleic acid binding.

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.

Enhanced translation of mRNAs encoding proteins involved in mRNA translation during recovery from heat shock

The mRNAs encoding poly (A) binding protein (PABP1), eukaryotic elongation factor 1A (eEF1A) and ribosomal protein S6 (RPS6) belong to the family of terminal oligo pyrimidine tract (TOP) containing mRNAs. Translation of the TOP mRNAs is regulated by growth signals and usually codes for proteins involved in mRNA translation. Previous studies from our laboratory showed that translation of PABP1 mRNA was preferentially enhanced during recovery of HeLa cells from heat shock. Presence of the 5′ TOP cis element was required for the observed increase of PABP1 mRNA translation. In the studies reported here we showed that translation of two additional TOP mRNAs such as, eEF1A and RPS6 was similarly enhanced during recovery. In addition, we showed by in vivo cross-linking experiments that the cellular nucleic acid binding protein ZNF9 binds to all three TOP mRNAs examined in these studies as well as to the β-actin mRNA that lacks a TOP cis element. Binding of ZNF9 to mRNAs was observed in both heat-shocked and non heat- shocked cells. However, depletion of ZNF9 by siRNA prevented the preferred stimulation of PABP1, eEF1A and RPS6 expression during recovery from heat shock. There was no detectable effect of ZNF9 depletion on the basal level of expression of either β-actin or PABP1, eEF1A and RPS6 in HeLa cells following recovery from heat shock.

Conclusion

Although the presence of ZNF9 was required for the translational stimulation of PABP1, eEF1A and RPS6 mRNAs, the mechanistic details of this process are still unclear. Since ZNF9 was shown to bind both TOP and non-TOP mRNAs, it is uncertain whether ZNF9 exerts its stimulatory effect on TOP mRNA translation following recovery from heat shock through the TOP cis-element. Perhaps additional factors or post-translational modification(s) of ZNF9 following heat shock are necessary for the preferred increase of TOP mRNA translation.