Oculopharyngeal muscular dystrophy (OPMD) is a late-onset autosomal dominant muscular dystrophy that results from small expansions of a polyalanine tract in the PABPN1 gene. Intranuclear inclusions are the pathological hallmark of OPMD. The mechanism by which protein aggregation in OPMD might relate to a toxic gain-of-function has so far remained elusive. Whether protein aggregates themselves are pathogenic or are the consequence of an unidentified underlying molecular mechanism is still unclear. Here, we report that protein aggregation in a cell model of OPMD directly impaires the function of the ubiquitin-proteasome pathway (UPP) as well as molecular chaperone functions. The proteasome inhibitor lactacystin causes significant increase of protein aggregation and toxicity. Moreover, overexpression of molecular chaperones (HSP40 and HSP70) suppressed protein aggregation and toxicity. We also provide evidence that mPABPN1-ala17 protein aggregation proportionally correlates with toxicity. Furthermore, we show that co-expression of chaperones in our OPMD cell model increases the solubility of mPABPN1-ala17 and transfected cell survival rate. Our studies suggest that molecular regulators of polyalanine protein solubility and degradation may provide insights into new mechanisms in OPMD pathogenesis. Further analysis of the cellular and molecular mechanisms by which UPP and molecular chaperones influence the degradation of misfolded proteins could provide novel concepts and targets for the treatment and understanding of the pathogenesis of OPMD and neurodegenerative diseases.

Oculopharyngeal muscular dystrophy (OPMD) is caused by polyalanine expansion in nuclear protein PABPN1 [poly(A) binding protein nuclear 1] and characterized by muscle degeneration. Druggable modifiers of proteotoxicity in degenerative diseases, notably the longevity modulators sirtuins, may constitute useful therapeutic targets. However, the modifiers of mutant PABPN1 are unknown. Here, we report that longevity and cell metabolism modifiers modulate mutant PABPN1 toxicity in the muscle cell. Using PABPN1 nematodes that show muscle cell degeneration and abnormal motility, we found that increased dosage of the sirtuin and deacetylase sir-2.1/SIRT1 exacerbated muscle pathology, an effect dependent on the transcription factor daf-16/FoxO and fuel sensor aak-2/AMPK (AMP-activated protein kinase), while null mutants of sir-2.1, daf-16 and aak-2 were protective. Consistently, the Sir2 inhibitor sirtinol was protective, whereas the Sir2 and AMPK activator resveratrol was detrimental. Furthermore, rescue by sirtinol was dependent on daf-16 and not aak-2, whereas aggravation by resveratrol was dependent on aak-2 and not daf-16. Finally, the survival of mammalian cells expressing mutant PABPN1 was promoted by sirtinol and decreased by resveratrol. Altogether, our data identify Sir2 and AMPK inhibition as therapeutic strategies for muscle protection in OPMD, extending the value of druggable proteins in cell maintenance networks to polyalanine diseases.

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disorder characterized by progressive eyelid drooping, swallowing difficulties and proximal limb weakness. OPMD is caused by a small expansion of a short polyalanine tract in the poly (A) binding protein nuclear 1 protein (PABPN1). The mechanism by which the polyalanine expansion mutation in PABPN1 causes disease is unclear. PABPN1 is a nuclear multi-functional protein which is involved in pre-mRNA polyadenylation, transcription regulation, and mRNA nucleocytoplasmic transport. The distinct pathological hallmark of OPMD is the presence of filamentous intranuclear inclusions (INIs) in patient's skeletal muscle cells. The exact relationship between mutant PABPN1 intranuclear aggregates and pathology is not clear. OPMD is a unique disease sharing common pathogenic features with other polyalanine disorders, as well as with polyglutamine and dystrophic disorders. This chapter aims to review the rapidly growing body of knowledge concerning OPMD. First, we outline the background of OPMD. Second, we compare OPMD with other trinucleotide repeat disorders. Third, we discuss the recent advances in the understanding of the molecular mechanisms underlying OPMD pathogenesis. Finally, we review recent therapeutic strategies for OPMD.

Expansion of polyalanine tracts causes at least nine inherited human diseases. Among these, a polyalanine tract expansion in the poly (A)-binding protein nuclear 1 (expPABPN1) causes oculopharyngeal muscular dystrophy (OPMD). So far, there is no treatment for OPMD patients. Developing drugs that efficiently sustain muscle protection by activating key cell survival mechanisms is a major challenge in OPMD research. Proteins that belong to the Wnt family are known for their role in both human development and adult tissue homeostasis. A hallmark of the Wnt signaling pathway is the increased expression of its central effector, beta-catenin (β-catenin) by inhibiting one of its upstream effector, glycogen synthase kinase (GSK)3β. Here, we explored a pharmacological manipulation of a Wnt signaling pathway using lithium chloride (LiCl), a GSK-3β inhibitor, and observed the enhanced expression of β-catenin protein as well as the decreased cell death normally observed in an OPMD cell model of murine myoblast (C2C12) expressing the expanded and pathogenic form of the expPABPN1. Furthermore, this effect was also observed in primary cultures of mouse myoblasts expressing expPABPN1. A similar effect on β-catenin was also observed when lymphoblastoid cells lines (LCLs) derived from OPMD patients were treated with LiCl. We believe manipulation of the Wnt/β-catenin signaling pathway may represent an effective route for the development of future therapy for patients with OPMD.