PABPN1 polyalanine tract deletion and long expansions modify its aggregation pattern and expression

Disruption of PABPN1 phase separation by SNRPD2 drives colorectal cancer cell proliferation and migration through promoting alternative polyadenylation of CTNNBIP1

Generally shortened 3' UTR due to alternative polyadenylation (APA) is widely observed in cancer, but its regulation mechanisms for cancer are not well characterized. Here, with profiling of APA in colorectal cancer tissues and poly(A) signal editing, we firstly identified that the shortened 3' UTR of CTNNIBP1 in colorectal cancer promotes cell proliferation and migration. We found that liquid-liquid phase separation (LLPS) of PABPN1 is reduced albeit with higher expression in cancer, and the reduction of LLPS leads to the shortened 3' UTR of CTNNBIP1 and promotes cell proliferation and migration. Notably, the splicing factor SNRPD2 upregulated in colorectal cancer, can interact with glutamic-proline (EP) domain of PABPN1, and then disrupt LLPS of PABPN1, which attenuates the repression effect of PABPN1 on the proximal poly(A) sites. Our results firstly reveal a new regulation mechanism of APA by disruption of LLPS of PABPN1, suggesting that regulation of APA by interfering LLPS of 3' end processing factor may have the potential as a new way for the treatment of cancer.

Oculopharyngeal muscular dystrophy mutations link the RNA-binding protein HNRNPQ to autophagosome biogenesis

Autophagy is an intracellular degradative process with an important role in cellular homeostasis. Here, we show that the RNA binding protein (RBP), heterogeneous nuclear ribonucleoprotein Q (HNRNPQ)/SYNCRIP is required to stimulate early events in autophagosome biogenesis, in particular the induction of VPS34 kinase by ULK1-mediated beclin 1 phosphorylation. The RBPs HNRNPQ and poly(A) binding protein nuclear 1 (PABPN1) form a regulatory network that controls the turnover of distinct autophagy-related (ATG) proteins. We also show that oculopharyngeal muscular dystrophy (OPMD) mutations engender a switch from autophagosome stimulation to autophagosome inhibition by impairing PABPN1 and HNRNPQ control of the level of ULK1. The overexpression of HNRNPQ in OPMD patient-derived cells rescues the defective autophagy in these cells. Our data reveal a regulatory mechanism of autophagy induction that is compromised by PABPN1 disease mutations, and may thus further contribute to their deleterious effects.

PABPN1 aggregation is driven by Ala expansion and poly(A)-RNA binding, leading to CFIm25 sequestration that impairs alternative polyadenylation

Poly(A)-binding protein nuclear 1 (PABPN1) is an RNA-binding protein localized in nuclear speckles, while its alanine (Ala)-expanded variants accumulate as intranuclear aggregates in oculopharyngeal muscular dystrophy. The factors that drive PABPN1 aggregation and its cellular consequences remain largely unknown. Here, we investigated the roles of Ala stretch and poly(A) RNA in the phase transition of PABPN1 using biochemical and molecular cell biology methods. We have revealed that the Ala stretch controls its mobility in nuclear speckles, and Ala expansion leads to aggregation from the dynamic speckles. Poly(A) nucleotide is essential to the early-stage condensation that thereby facilitates speckle formation and transition to solid-like aggregates. Moreover, the PABPN1 aggregates can sequester CFIm25, a component of the pre-mRNA 3'-UTR processing complex, in an mRNA-dependent manner and consequently impair the function of CFIm25 in alternative polyadenylation. In conclusion, our study elucidates a molecular mechanism underlying PABPN1 aggregation and sequestration, which will be beneficial for understanding PABPN1 proteinopathy.

Abnormal phase separation of biomacromolecules in human diseases

Membrane-less organelles (MLOs) formed through liquid-liquid phase separation (LLPS) are associated with numerous important biological functions, but the abnormal phase separation will also dysregulate the physiological processes. Emerging evidence points to the importance of LLPS in human health and diseases. Nevertheless, despite recent advancements, our knowledge of the molecular relationship between LLPS and diseases is frequently incomplete. In this review, we outline our current understanding about how aberrant LLPS affects developmental disorders, tandem repeat disorders, cancers and viral infection. We also examine disease mechanisms driven by aberrant condensates, and highlight potential treatment approaches. This study seeks to expand our understanding of LLPS by providing a valuable new paradigm for understanding phase separation and human disorders, as well as to further translate our current knowledge regarding LLPS into therapeutic discoveries.

The length of FOXE1 polyalanine tract in congenital hypothyroidism: Evidence for a pathogenic role from familial, molecular and cohort studies

INTRODUCTION

FOXE1 is required for thyroid function and its homozygous mutations cause a rare syndromic form of congenital hypothyroidism (CH). FOXE1 has a polymorphic polyalanine tract whose involvement in thyroid pathology is controversial. Starting from genetic studies in a CH family, we explored the functional role and involvement of FOXE1 variations in a large CH population.

METHODS

We applied NGS screening to a large CH family and a cohort of 1752 individuals and validated these results by in silico modeling and in vitro experiments.

RESULTS

A new heterozygous FOXE1 variant segregated with 14-Alanine tract homozygosity in 5 CH siblings with athyreosis. The p.L107V variant demonstrated to significantly reduce the FOXE1 transcriptional activity. The 14-Alanine-FOXE1 displayed altered subcellular localization and significantly impaired synergy with other transcription factors, when compared with the more common 16-Alanine-FOXE1. The CH group with thyroid dysgenesis was largely and significantly enriched with the 14-Alanine-FOXE1 homozygosity.

DISCUSSION

We provide new evidence that disentangle the pathophysiological role of FOXE1 polyalanine tract, thereby significantly broadening the perspective on the role of FOXE1 in the complex pathogenesis of CH. FOXE1 should be therefore added to the group of polyalanine disease-associated transcription factors.

Alternative polyadenylation utilization results in ribosome assembly and mRNA translation deficiencies in a model for muscle aging

Aging-associated muscle wasting is regulated by multiple molecular processes, whereby aberrant mRNA processing regulation induces muscle wasting. The poly(A)-binding protein nuclear 1 (PABPN1) regulates polyadenylation site (PAS) utilization, in the absence of PABPN1 the alternative PAS (APA) is utilized. Reduced PABPN1 levels induce muscle wasting where the expression of cellular processes regulating protein homeostasis, the ubiquitin-proteasome system, and translation, are robustly dysregulated. Translation is impacted by mRNA levels, but PABPN1 impact on translation is not fully understood. Here we show that a persistent reduction in PABPN1 levels led to a significant loss of translation efficiency. RNA sequencing of rRNA-depleted libraries from polysome traces revealed reduced mRNA abundance across ribosomal fractions, as well as reduced levels of small RNAs. We show that the abundance of translated mRNAs in the polysomes correlated with PAS switches at the 3'-UTR. Those mRNAs are enriched in cellular processes that are essential for proper muscle function. This study suggests that the effect of PABPN1 on translation efficiency impacts protein homeostasis in aging-associated muscle atrophy.

Immortalized human myotonic dystrophy muscle cell lines to assess therapeutic compounds

Myotonic dystrophy type 1 (DM1) and type 2 (DM2) are autosomal dominant neuromuscular diseases caused by microsatellite expansions and belong to the family of RNA-dominant disorders. Availability of cellular models in which the DM mutation is expressed within its natural context is essential to facilitate efforts to identify new therapeutic compounds. Here, we generated immortalized DM1 and DM2 human muscle cell lines that display nuclear RNA aggregates of expanded repeats, a hallmark of myotonic dystrophy. Selected clones of DM1 and DM2 immortalized myoblasts behave as parental primary myoblasts with a reduced fusion capacity of immortalized DM1 myoblasts when compared with control and DM2 cells. Alternative splicing defects were observed in differentiated DM1 muscle cell lines, but not in DM2 lines. Splicing alterations did not result from differentiation delay because similar changes were found in immortalized DM1 transdifferentiated fibroblasts in which myogenic differentiation has been forced by overexpression of MYOD1. As a proof-of-concept, we show that antisense approaches alleviate disease-associated defects, and an RNA-seq analysis confirmed that the vast majority of mis-spliced events in immortalized DM1 muscle cells were affected by antisense treatment, with half of them significantly rescued in treated DM1 cells. Immortalized DM1 muscle cell lines displaying characteristic disease-associated molecular features such as nuclear RNA aggregates and splicing defects can be used as robust readouts for the screening of therapeutic compounds. Therefore, immortalized DM1 and DM2 muscle cell lines represent new models and tools to investigate molecular pathophysiological mechanisms and evaluate the in vitro effects of compounds on RNA toxicity associated with myotonic dystrophy mutations.

Summary: Myotonic dystrophy muscle cell models displaying characteristic disease-associated molecular features can be used to investigate molecular pathophysiological mechanisms and evaluate therapeutic approaches.

An Antibody to Detect Alanine-Expanded PABPN1: A New Tool to Study Oculopharyngeal Muscular Dystrophy

BACKGROUND

Oculopharyngeal muscular dystrophy (OPMD), a late onset disorder affecting specific skeletal muscles, is caused by a (GCG)n expansion mutation in the gene encoding the mRNA processing protein, polyadenylate binding protein nuclear 1 (PABPN1). The expansion in PABPN1 leads to an increase in a stretch of N-terminal alanine residues in the PABPN1 protein from the normal 10 to 12-18. Given this modest change, detection of mutant protein has not been possible without the use of tagged constructs.

OBJECTIVE

We sought to generate a polyclonal antibody that recognizes alanine-expanded but not wild type PABPN1 with the goal of making possible analysis of expression and localization of alanine-expanded PABPN1.

METHODS

We immunized rabbits with a GST-tagged alanine peptide and tested the resulting serum against alanine-expanded PABPN1 expressed in cell culture as well as in animal models of OPMD.

RESULTS

The resulting α-alanine antibody detected PABPN1 proteins that contained 14 or more alanine residues. Importantly, the α-alanine antibody could be used to detect alanine-expanded PABPN1 in muscles from either a mouse or Drosophila model of OPMD.

CONCLUSIONS

This α-alanine antibody provides a new tool that will allow for more in-depth study of how alanine expansion affects aggregation, localization, and steady-state levels of alanine-expanded PABPN1 levels in vivo, providing new insight into the molecular mechanisms underlying OPMD.

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.

Polyalanine tract disorders and neurocognitive phenotypes

Expansion of polyalanine tracts cause at least 9 inherited human diseases. Eight of these nine diseases are due to expansions in transcription factors and give rise to congenital disorders, many with neurocognitive phenotypes. Disease-causing expansions vary in length dependingupon the gene in question, with the severity of the associated clinical phenotype generally increasing with length of the polyalanine tract. The past decade has seen considerable progress in the understanding on how these mutations may arise and the functional effect of expanded polyalanine tracts on the resulting protein. Despite this progress, the pathogenic mechanism of expanded polyalanine tracts contributing to the associated disease states remains poorly understood. Gaining insights into the mechanisms that underlie the pathogenesis of different expanded polyalanine tract mutations will be a necessary step on the path to the design of potential treatment strategies for the associated diseases.

Expression of the polyalanine expansion mutant of nuclear poly(A)-binding protein induces apoptosis via the p53 pathway

The PABPN1 [nuclear poly(A)-binding protein 1] is ubiquitous, binds to the nascent mRNA transcript and controls the poly(A) tract elongation process in multicellular organisms. Expansion of GCG repeats that encode first 6 of the 10 alanine residues of a polyalanine tract at the N-terminus of wild-type PABPN1 to 12-17 alanine residues causes aggregation of the protein and cell death. Patients with the adult onset autosomal dominant OPMD (oculopharyngeal muscular dystrophy) carry the GCG expansion mutation in their PABPN1 gene. The symptoms of OPMD include drooping eye lids and difficulty swallowing. The severity of symptoms increases with the length of the expansion. We have investigated the mechanism of cell death in HeLa and HEK-293 (human embryonic kidney) cultured cells expressing the mutant PABPN1 with a polyalanine tract containing 17 alanine residues (PABPN1-A17). In cells expressing PABPN1-A17, the abundance of pro-apoptotic proteins, p53, PUMA (p53 up-regulated modulator of apoptosis) and Noxa, are up-regulated. This was associated with the redistribution of p53 to the nucleus and mitochondria. Concomitantly Bax was translocated to the mitochondria, followed by the release of cytochrome c and the cleavage of caspase 3. Furthermore, blocking p53-mediated transcription using pifithrin significantly reduced apoptosis. Our findings suggest a key role of p53-mediated apoptosis in death of cells expressing the polyalanine expansion mutant of PABPN1.

Reversible aggregation of PABPN1 pre-inclusion structures

Increased aggregation of misfolded proteins is associated with aging, and characterizes a number of neurodegenerative disorders caused by homopolymeric amino acid expansion mutations. PABPN1 is an aggregation-prone nuclear protein. Natural aggregation of wild-type (WT) PABPN1 is not known to be disease-associated, but alanine-expanded PABPN1 (expPABPN1) accumulates in insoluble intranuclear inclusions in muscle of patients with oculopharyngeal muscular dystrophy (OPMD). We applied microscopic image quantification to study PABPN1 aggregation process in living cells. We identified transitional pre-inclusion foci and demonstrate that these structures significantly differ between WT- and expPABPN1-expressing cells, while inclusions of these proteins are indistinguishable. In addition to the immobile PABPN1 in inclusions, in the nucleoplasm of expPABPN1 expressing cells we also found a fraction of immobile proteins, representing pre-aggregated species. We found that pre-aggregated and pre-inclusion structures are reverted by a PABPN1 specific affinity binder while inclusion structures are not. Together our results demonstrate that the aggregation process of WT- and expPABPN1 differs in steps preceding inclusion formation, suggesting that pre-aggregated protein species could represent the cytotoxic structures.

The E3 ubiquitin ligase TRIM11 mediates the degradation of congenital central hypoventilation syndrome-associated polyalanine-expanded PHOX2B

Expansions of a polyalanine (polyA) stretch in the coding region of the PHOX2B gene cause congenital central hypoventilation syndrome (CCHS), a neurocristopathy characterized by the absence of adequate control of autonomic breathing. Expansion of polyA in PHOX2B leads to protein misfolding and accumulation into inclusions. The mechanisms that regulate mutant protein degradation and turnover have been poorly elucidated. Here, we investigate the regulation of degradation of wild-type and polyA-expanded PHOX2B. We show that expanded PHOX2B is targeted for degradation through the ubiquitin-proteasome system, resulting in lowered levels of the mutant protein relative to its wild-type counterpart. Moreover, we show that mutant PHOX2B forms ubiquitin-positive inclusions, which sequester wild-type PHOX2B. This sequestration correlates with reduced transcriptional activity of endogenous wild-type protein in neuroblastoma cells. Finally, we show that the E3 ubiquitin ligase TRIM11 plays a critical role in the clearance of mutant PHOX2B through the proteasome. Importantly, clearance of mutant PHOX2B by TRIM11 correlates with a rescue of PHOX2B transcriptional activity. We propose that CCHS is partially caused by a dominant-negative effect of expanded PHOX2B due to the retention of the wild-type protein in pathogenic aggregates. Our results demonstrate that TRIM11 is a novel modifier of mutant PHOX2B toxicity and represents a potential therapeutic target for CCHS.

Nuclear speckles are involved in nuclear aggregation of PABPN1 and in the pathophysiology of oculopharyngeal muscular dystrophy

Nuclear speckles are essential nuclear compartments involved in the assembly, delivery and recycling of pre-mRNA processing factors, and in the post-transcriptional processing of pre-mRNAs. Oculopharyngeal muscular dystrophy (OPMD) is caused by a small expansion of the polyalanine tract in the poly(A)-binding protein nuclear 1 (PABPN1). Aggregation of expanded PABPN1 into intranuclear inclusions (INIs) in skeletal muscle fibers is the pathological hallmark of OPMD. In this study what we have analyzed in muscle fibers of OPMD patients and in primary cultures of human myoblasts are the relationships between nuclear speckles and INIs, and the contribution of the former to the biogenesis of the latter. While nuclear speckles concentrate snRNP splicing factors and PABPN1 in control muscle fibers, they are depleted of PABPN1 and appear closely associated with INIs in muscle fibers of OPMD patients. The induction of INI formation in human myoblasts expressing either wild type GFP-PABPN1 or expanded GFP-PABPN1-17ala demonstrates that the initial aggregation of PABPN1 proteins and their subsequent growth in INIs occurs at the edges of the nuclear speckles. Moreover, the growing of INIs gradually depletes PABPN1 proteins and poly(A) RNA from nuclear speckles, although the existence of these nuclear compartments is preserved. Time-lapse experiments in cultured myoblasts confirm nuclear speckles as biogenesis sites of PABPN1 inclusions. Given the functional importance of nuclear speckles in the post-transcriptional processing of pre-mRNAs, the INI-dependent molecular reorganization of these nuclear compartments in muscle fibers may cause a severe dysfunction in nuclear trafficking and processing of polyadenylated mRNAs, thereby contributing to the molecular pathophysiology of OPMD. Our results emphasize the potential importance of nuclear speckles as nuclear targets of neuromuscular disorders.

Modeling oculopharyngeal muscular dystrophy in myotube cultures reveals reduced accumulation of soluble mutant PABPN1 protein

Oculopharyngeal muscular dystrophy (OPMD) is an autosomal dominant disease caused by an alanine tract expansion mutation in poly(A) binding protein nuclear 1 (expPABPN1). To model OPMD in a myogenic and physiological context, we generated mouse myoblast cell clones stably expressing either human wild type (WT) or expPABPN1 at low levels. Transgene expression is induced on myotube differentiation and results in formation of insoluble nuclear PABPN1 aggregates that are similar to those observed in patients with OPMD. Quantitative analysis of PABPN1 in myotube cultures revealed that expPABPN1 accumulation and aggregation is greater than that of the WT protein. We found that aggregation of expPABPN1 is more affected than WT PABPN1 by inhibition of proteasome activity. Consistent with this, in myotube cultures expressing expPABPN1, deregulation of the proteasome was identified as the most significantly perturbed pathway. Differences in the accumulation of soluble WT and expPABPN1 were consistent with differences in ubiquitination and rate of protein turnover. This study demonstrates, for the first time to our knowledge, that, in myotubes, the ratio of soluble/insoluble expPABPN1 is significantly lower compared with that of the WT protein. We suggest that this difference can contribute to muscle weakness in OPMD.

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.

A yeast model for polyalanine-expansion aggregation and toxicity

Polyalanine expansions can result in aggregation and cause cytotoxicity. We have created the first yeast model of polyalanine-expansion aggregation and toxicity using the poly(Ade)-binding protein Pab1.

Nine human disorders result from the toxic accumulation and aggregation of proteins with expansions in their endogenous polyalanine (polyA) tracts. Given the prevalence of polyA tracts in eukaryotic proteomes, we wanted to understand the generality of polyA-expansion cytotoxicity by using yeast as a model organism. In our initial case, we expanded the polyA tract within the native yeast poly(Adenine)-binding protein Pab1 from 8A to 13A, 15A, 17A, and 20A. These expansions resulted in increasing formation of Pab1 inclusions, insolubility, and cytotoxicity that correlated with the length of the polyA expansion. Pab1 binds mRNA as part of its normal function, and disrupting RNA binding or altering cytoplasmic mRNA levels suppressed the cytotoxicity of 17A-expanded Pab1, indicating a requisite role for mRNA in Pab1 polyA-expansion toxicity. Surprisingly, neither manipulation suppressed the cytotoxicity of 20A-expanded Pab1. Thus longer expansions may have a different mechanism for toxicity. We think that this difference underscores the potential need to examine the cytotoxic mechanisms of both long and short expansions in models of expansion disorders.

Contractions in the second polyA tract of ARX are rare, non-pathogenic polymorphisms

Aristaless related homeobox (ARX) is a transcription factor containing highly conserved octapeptide, homeobox, acidic, and aristaless domains, as well as four polyA tracts. The most frequent ARX mutation found to date in patients with X-linked infantile spasms, Partington syndrome or X-linked mental retardation, is a duplication of 24 bp in exon 2, resulting in the expansion of the second polyA tract. Although the pathogenic role of this expansion has been well characterized, the effect of contractions in the same polyA tract is still debated since different reports have associated contractions to either mental retardation or a normal phenotype. Here, we report two unrelated girls with epilepsy and mental retardation who inherited from their unaffected parents, of either sex, a deletion of 24 bp (c.441_464del), resulting in a contraction of eight alanines in the second polyA tract of ARX. Segregation studies revealed the c.441_464del also in two healthy relatives of one of the patients. This finding supports the hypothesis that this contraction represents a rare, benign polymorphism.

Modeling human diseases in Caenorhabditis elegans

Genes linked to human diseases often function in evolutionarily conserved pathways, which can be readily dissected in simple model organisms. Because of its short lifespan and well-known biology, coupled with a completely sequenced genome that shares extensive homology with that of mammals, Caenorhabditis elegans is one of the most versatile and powerful model organisms. Research in C. elegans has been instrumental for the elucidation of molecular pathways implicated in many human diseases. In this review, we introduce C. elegans as a model organism for biomedical research and we survey recent relevant findings that shed light on the basic molecular determinants of human disease pathophysiology. The nematode holds promise of providing clear leads towards the identification of potential targets for the development of new therapeutic interventions against human diseases.

Cross-talk between canonical Wnt signaling and the sirtuin-FoxO longevity pathway to protect against muscular pathology induced by mutant PABPN1 expression in C. elegans

Developmental pathways may be play a role in adult cell survival. However, whether they interact with longevity/cell survival pathways to confer protection against disease-associated proteotoxicity remains largely unknown. We previously reported that the inhibition of key longevity modulators such as the deacetylase sir-2.1/SIRT1 (Sir2) and its target daf-16/FoxO protects transgenics nematodes from muscle cell decline and abnormal motility produced by the expression of mutant (polyalanine-expanded) PABPN1, the oculopharyngeal muscular dystrophy (OPMD) protein. Here, we report that canonical Wnt signaling (i) modulates muscular pathology in mutant PABPN1 nematodes, and (ii) cooperates with the Sir2-FoxO longevity pathway to confer protection against mutant PABPN1 toxicity at the cellular and behavioral levels. Mutant PABPN1 toxicity was modified by genes along the canonical Wnt pathway, several of which depend on daf-16 for activity. ss-catenin and pop-1/TCF RNAi suppressed the protection from mutant PABPN1 confered by loss-of-function mutations in sir-2.1 and daf-16. Moreover, the aggravation of muscle cell pathology by increased sir-2.1 dosage was reversed by ss-catenin and pop-1 RNAi. The chemical inhibition of GSK-3ss, a repressor of ss-catenin activity, protected against mutant PABPN1 toxicity in a daf-16-dependent manner, which is consistent with a cross-talk between ss-catenin signaling and Sir2-FoxO signaling in protecting from mutant PABPN1 toxicity. Our data reveal that canonical Wnt signaling and Sir2-FoxO signaling interact to modulate diseased muscle survival, and indicate that GSK-3ss inhibitors and sirtuin inhibitors both have therapeutic potential for muscle protection in OPMD.

Molecular and phenotypic characterization of a mouse model of oculopharyngeal muscular dystrophy reveals severe muscular atrophy restricted to fast glycolytic fibres

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disorder characterized by ptosis, dysphagia and proximal limb weakness. Autosomal-dominant OPMD is caused by a short (GCG)(8-13) expansions within the first exon of the poly(A)-binding protein nuclear 1 gene (PABPN1), leading to an expanded polyalanine tract in the mutated protein. Expanded PABPN1 forms insoluble aggregates in the nuclei of skeletal muscle fibres. In order to gain insight into the different physiological processes affected in OPMD muscles, we have used a transgenic mouse model of OPMD (A17.1) and performed transcriptomic studies combined with a detailed phenotypic characterization of this model at three time points. The transcriptomic analysis revealed a massive gene deregulation in the A17.1 mice, among which we identified a significant deregulation of pathways associated with muscle atrophy. Using a mathematical model for progression, we have identified that one-third of the progressive genes were also associated with muscle atrophy. Functional and histological analysis of the skeletal muscle of this mouse model confirmed a severe and progressive muscular atrophy associated with a reduction in muscle strength. Moreover, muscle atrophy in the A17.1 mice was restricted to fast glycolytic fibres, containing a large number of intranuclear inclusions (INIs). The soleus muscle and, in particular, oxidative fibres were spared, even though they contained INIs albeit to a lesser degree. These results demonstrate a fibre-type specificity of muscle atrophy in this OPMD model. This study improves our understanding of the biological pathways modified in OPMD to identify potential biomarkers and new therapeutic targets.

Loss of nuclear poly(A)-binding protein 1 causes defects in myogenesis and mRNA biogenesis

The nuclear poly(A)-binding protein 1 (PABPN1) is a ubiquitously expressed protein that plays a critical role in polyadenylation. Short expansions of the polyalanine tract in the N-terminus of PABPN1 lead to oculopharyngeal muscular dystrophy (OPMD), which is an adult onset disease characterized by eyelid drooping, difficulty in swallowing and weakness in the proximal limb muscles. Although significant data from in vitro biochemical assays define the function of PABPN1 in control of poly(A) tail length, little is known about the role of PABPN1 in mammalian cells. To assess the function of PABPN1 in mammalian cells and specifically in cells affected in OPMD, we examined the effects of PABPN1 depletion using siRNA in primary mouse myoblasts from extraocular, pharyngeal and limb muscles. PABPN1 knockdown significantly decreased cell proliferation and myoblast differentiation during myogenesis in vitro. At the molecular level, PABPN1 depletion in myoblasts led to a shortening of mRNA poly(A) tails, demonstrating the cellular function of PABPN1 in polyadenylation control in a mammalian cell. In addition, PABPN1 depletion caused nuclear accumulation of poly(A) RNA, revealing that PABPN1 is required for proper poly(A) RNA export from the nucleus. Together, these experiments demonstrate that PABPN1 plays an essential role in myoblast proliferation and differentiation, suggesting that it is required for muscle regeneration and maintenance in vivo.

Hsp70 chaperones and type I PRMTs are sequestered at intranuclear inclusions caused by polyalanine expansions in PABPN1

Genomic instability at loci with tandem arrays of simple repeats is the cause for many neurological, neurodegenerative and neuromuscular diseases. When located in coding regions, disease-associated expansions of trinucleotide repeats are translated into homopolymeric amino acid stretches of glutamine or alanine. Polyalanine expansions in the poly(A)-binding protein nuclear 1 (PABPN1) gene causes oculopharyngeal muscular dystrophy (OPMD). To gain novel insight into the molecular pathophysiology of OPMD, we studied the interaction of cellular proteins with normal and expanded PABPN1. Pull-down assays show that heat shock proteins including Hsp70, and type I arginine methyl transferases (PRMT1 and PRMT3) associate preferentially with expanded PABPN1. Immunofluorescence microscopy further reveals accumulation of these proteins at intranuclear inclusions in muscle from OPMD patients. Recombinant PABPN1 with expanded polyalanine stretches binds Hsp70 with higher affinity, and data from molecular simulations suggest that expansions of the PABPN1 polyalanine tract result in transition from a disordered, flexible conformation to a stable helical secondary structure. Taken together, our results suggest that the pathological mutation in the PABPN1 gene alters the protein conformation and induces a preferential interaction with type I PRMTs and Hsp70 chaperones. This in turn causes sequestration in intranuclear inclusions, possibly leading to a progressive cellular defect in arginine methylation and chaperone activity.

Oculopharyngeal muscular dystrophy: a polyalanine myopathy

It has been 10 years since the identification of the first PABPN1 gene (GCN)(n)/polyalanine mutations responsible for oculopharyngeal muscular dystrophy (OPMD). These mutations have been found in most cases of OPMD diagnosed in more than 35 countries. Sequence analyses have shown that such mutations have occurred numerous times in human history. Although PABPN1 was found early on to be a component of the classic filamentous intranuclear inclusions (INIs), mRNA and other proteins also have been found to coaggregate in the INIs. It is still unclear if the INIs play a pathologic or a protective role. The generation of numerous cell and animal models of OPMD has led to greater insight into its complex molecular pathophysiology and identified the first candidate therapeutic molecules. This paper reviews basic and clinical research on OPMD, with special emphasis on recent developments in the understanding of its pathophysiology.