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

Caenorhabditis elegans as a model system for human diseases

The nematode Caenorhabditis elegans offers unique advantages that enable a comprehensive delineation of the cellular and molecular mechanisms underlying devastating human pathologies such as stroke, ischemia and age-associated neurodegenerative disorders. Genetic models of human diseases that closely simulate several disease-related phenotypes have been established in the worm. These models allow the implementation of multidisciplinary approaches, in addition to large-scale genetic and pharmacological screenings, designed to elucidate the molecular mechanisms mediating pathogenesis and to identify targets and drugs for emergent therapeutic interventions. Such strategies have already provided valuable insights, highly relevant to human health and quality of life. This article considers the potential of C. elegans as a versatile platform for systematic dissection of the molecular basis of human disease, focusing on neurodegenerative disorders.

Progress on gene therapy, cell therapy, and pharmacological strategies toward the treatment of oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a muscle-specific, late-onset degenerative disorder whereby muscles of the eyes (causing ptosis), throat (leading to dysphagia), and limbs (causing proximal limb weakness) are mostly affected. The disease is characterized by a mutation in the poly(A)-binding protein nuclear-1 (PABPN1) gene, resulting in a short GCG expansion in the polyalanine tract of PABPN1 protein. Accumulation of filamentous intranuclear inclusions in affected skeletal muscle cells constitutes the pathological hallmark of OPMD. This review highlights the current translational research advances in the treatment of OPMD. In vitro and in vivo disease models are described. Conventional and experimental therapeutic approaches are discussed with emphasis on novel molecular therapies including the use of intrabodies, gene therapy, and myoblast transfer therapy.

Inactivation of glycogen synthase kinase-3β up-regulates β-catenin and promotes chondrogenesis

This study aimed to investigate whether inhibition of glycogen synthase kinase-3β (GSK-3β) could promote chondrocytes proliferation. The expression pattern of GSK-3β was firstly determined by immunohistochemistry (IHC) in normal mouse. Tibias were then isolated and cultured for 6 days. The tibias were treated with dimethylsulfoxide (control) or GSK-3 inhibitor SB415286 (SB86). Length of tibias was measured until 6 days after treatment. These bones were either stained with alcian blue/alizarin red or analyzed by IHC. In addition, GSK-3β and β-catenin were analyzed by Western blot. Finally, cartilage-specific GSK-3β deletion mice (KO) were generated. Efficiency of GSK-3β deletion was determined through Western blot and IHC. After treated by inhibitor SB86, the overall length of growth plate was not changed. However, growth of tibia in SB86 group was increased by 31 %, the length of resting and proliferating was increased 13 % (P < 0.01), whereas the length of hypertrophic was decreased by 57 % (P < 0.01). Besides, the mineralized length was found to be significant longer than the control group (P < 0.05). In KO mice, growth plate and calvaria tissue both exhibit significant reduction of GSK-3β (P < 0.05) whereas the lengths of tibias in KO were almost same compared with control mice. Finally, an increase amount of β-catenin protein was observed in SB86 (P < 0.05). In addition, significantly increased β-catenin was also found in the growth plate of KO mice (P < 0.05). Inhibition of GSK-3 could promote longitudinal growth of bone through increasing bone formation. Besides, the inactivation of GSK-3β could lead to enhancing β-catenin, therefore promote chondrocytes proliferation.

Animal models in therapeutic drug discovery for oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy (OPMD) is a late onset disease which affects specific muscles. No pharmacological treatments are currently available for OPMD. In recent years, genetically tractable models of OPMD – Drosophila and Caenorhabditis elegans – have been generated. Although these models have not yet been used for large-scale primary drug screening, they have been very useful in candidate approaches for the identification of potential therapeutic compounds for OPMD. In this brief review, we summarize the data that validated active molecules for OPMD in animal models including Drosophila, C. elegans and mouse.

Wnt pathway regulation by long-term moderate exercise in rat hippocampus

An active lifestyle involving regular exercise reduces the deleterious effects of the aging process. At the cerebral level, both synaptic plasticity and neurogenesis are modulated by exercise, although the molecular mechanisms underlying these effects are not clearly understood. In the mature nervous system, the canonical Wnt (Wnt/β-catenin) signaling pathway is implicated in neuroprotection and synaptic plasticity. Here, we examined whether the Wnt pathway could be modulated in adult male rat hippocampus by long-term moderate exercise (treadmill running) or enrichment (handling/environmental stimulation). Sedentary animals showed higher protein levels of the Wnt antagonist, Dkk-1, the lowest levels being found in the exercised group. Although there was no evidence of any changes in activation of the LRP6 receptor, the total levels of LRP6 were higher in exercised and enriched animals. Analysis of some of the components implicated in the phosphorylation of β-catenin, which leads ultimately to its proteasomal degradation, revealed higher levels and activation of Axin1 and GSK-3α/β respectively in sedentary animals. However neither different phosphorylated forms nor total β-catenin protein levels differed between the experimental groups. Higher protein levels of Axin2 and the antiapoptotic protein, Bcl-2, were found with exercise and handling, whereas the proapototic, Bax, was unaffected. Thus, our results suggest activation of the Wnt pathway not only with moderate exercise, but also with the handling of the animals.

Lithium chloride attenuates cell death in oculopharyngeal muscular dystrophy by perturbing Wnt/β-catenin pathway

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.

Pathways to decoding the clinical potential of stress response FOXO-interaction networks for Huntington's disease: of gene prioritization and context dependence

The FOXO family of transcription factors is central to the regulation of organismal longevity and cellular survival. Several studies have indicated that FOXO factors lie at the center of a complex network of upstream pathways, cofactors and downstream targets (FOXO-interaction networks), which may have developmental and post-developmental roles in the regulation of chronic-stress response in normal and diseased cells. Noticeably, FOXO factors are important for the regulation of proteotoxicity and neuron survival in several models of neurodegenerative disease, suggesting that FOXO-interaction networks may have therapeutic potential. However, the status of FOXO-interaction networks in neurodegenerative disease remains largely unknown. Systems modeling is anticipated to provide a comprehensive assessment of this question. In particular, interrogating the context-dependent variability of FOXO-interaction networks could predict the clinical potential of cellular-stress response genes and aging regulators for tackling brain and peripheral pathology in neurodegenerative disease. Using published transcriptomic data obtained from murine models of Huntington's disease (HD) and post-mortem brains, blood samples and induced-pluripotent-stem cells from HD carriers as a case example, this review briefly highlights how the biological status and clinical potential of FOXO-interaction networks for HD may be decoded by developing network and entropy based feature selection across heterogeneous datasets.

PABPN1: molecular function and muscle disease

The polyadenosine RNA binding protein polyadenylate-binding nuclear protein 1 (PABPN1) plays key roles in post-transcriptional processing of RNA. Although PABPN1 is ubiquitously expressed and presumably contributes to control of gene expression in all tissues, mutation of the PABPN1 gene causes the disease oculopharyngeal muscular dystrophy (OPMD), in which a limited set of skeletal muscles are affected. A major goal in the field of OPMD research is to understand why mutation of a ubiquitously expressed gene leads to a muscle-specific disease. PABPN1 plays a well-documented role in controlling the poly(A) tail length of RNA transcripts but new functions are emerging through studies that exploit a variety of unbiased screens as well as model organisms. This review addresses (a) the molecular function of PABPN1 incorporating recent findings that reveal novel cellular functions for PABPN1 and (b) the approaches that are being used to understand the molecular defects that stem from expression of mutant PABPN1. The long-term goal in this field of research is to understand the key molecular functions of PABPN1 in muscle as well as the mechanisms that underlie the pathological consequences of mutant PABPN1. Armed with this information, researchers can seek to develop therapeutic approaches to enhance the quality of life for patients afflicted with OPMD.

C. elegans SIRT6/7 homolog SIR-2.4 promotes DAF-16 relocalization and function during stress

FoxO transcription factors and sirtuin family deacetylases regulate diverse biological processes, including stress responses and longevity. Here we show that the Caenorhabditis elegans sirtuin SIR-2.4—homolog of mammalian SIRT6 and SIRT7 proteins—promotes DAF-16–dependent transcription and stress-induced DAF-16 nuclear localization. SIR-2.4 is required for resistance to multiple stressors: heat shock, oxidative insult, and proteotoxicity. By contrast, SIR-2.4 is largely dispensable for DAF-16 nuclear localization and function in response to reduced insulin/IGF-1-like signaling. Although acetylation is known to regulate localization and activity of mammalian FoxO proteins, this modification has not been previously described on DAF-16. We find that DAF-16 is hyperacetylated in sir-2.4 mutants. Conversely, DAF-16 is acetylated by the acetyltransferase CBP-1, and DAF-16 is hypoacetylated and constitutively nuclear in response to cbp-1 inhibition. Surprisingly, a SIR-2.4 catalytic mutant efficiently rescues the DAF-16 localization defect in sir-2.4 null animals. Acetylation of DAF-16 by CBP-1 in vitro is inhibited by either wild-type or mutant SIR-2.4, suggesting that SIR-2.4 regulates DAF-16 acetylation indirectly, by preventing CBP-1-mediated acetylation under stress conditions. Taken together, our results identify SIR-2.4 as a critical regulator of DAF-16 specifically in the context of stress responses. Furthermore, they reveal a novel role for acetylation, modulated by the antagonistic activities of CBP-1 and SIR-2.4, in modulating DAF-16 localization and function.

Sensing and responding appropriately to environmental insults is a challenge facing all organisms. In the roundworm C. elegans, the FoxO protein DAF-16 moves to the nucleus in response to stress, where it regulates gene expression and plays a key role in ensuring organismal survival. In this manuscript, we characterize SIR-2.4 as a novel factor that promotes DAF-16 function during stress. SIR-2.4 is a member of a family of proteins called sirtuins, some of which promote increased lifespan in model organisms. Worms lacking SIR-2.4 show impaired DAF-16 nuclear recruitment, DAF-16–dependent gene expression, and survival in response to a variety of stressors. SIR-2.4 regulates DAF-16 by indirectly affecting levels of a modification called acetylation on DAF-16. Overall, our work has revealed SIR-2.4 to be a key new factor in stress resistance and DAF-16 regulation in C. elegans. Future studies will address whether mammalian SIR-2.4 homologs SIRT6 and SIRT7 act similarly towards mammalian FoxO proteins.

The alternative splice variant of protein tyrosine kinase 6 negatively regulates growth and enhances PTK6-mediated inhibition of β-catenin

Protein tyrosine kinase 6 (PTK6), also called breast tumor kinase (BRK), is expressed in epithelial cells of various tissues including the prostate. Previously it was shown that PTK6 is localized to epithelial cell nuclei in normal prostate, but becomes cytoplasmic in human prostate tumors. PTK6 is also primarily cytoplasmic in the PC3 prostate adenocarcinoma cell line. Sequencing revealed expression of wild type full-length PTK6 transcripts in addition to an alternative transcript lacking exon 2 in PC3 cells. The alternative transcript encodes a 134 amino acid protein, referred to here as ALT-PTK6, which shares the first 77 amino acid residues including the SH3 domain with full length PTK6. RT-PCR was used to show that ALT-PTK6 is coexpressed with full length PTK6 in established human prostate and colon cell lines, as well as in primary cell lines derived from human prostate tissue and tumors. Although interaction between full-length PTK6 and ALT-PTK6 was not detected, ALT-PTK6 associates with the known PTK6 substrates Sam68 and β-catenin in GST pull-down assays. Coexpression of PTK6 and ALT-PTK6 led to suppression of PTK6 activity and reduced association of PTK6 with tyrosine phosphorylated proteins. While ALT-PTK6 alone did not influence β-catenin/TCF transcriptional activity in a luciferase reporter assay, it enhanced PTK6-mediated inhibition of β-catenin/TCF transcription by promoting PTK6 nuclear functions. Ectopic expression of ALT-PTK6 led to reduced expression of the β-catenin/TCF targets Cyclin D1 and c-Myc in PC3 cells. Expression of tetracycline-inducible ALT-PTK6 blocked the proliferation and colony formation of PC3 cells. Our findings suggest that ALT-PTK6 is able to negatively regulate growth and modulate PTK6 activity, protein-protein associations and/or subcellular localization. Fully understanding functions of ALT-PTK6 and its impact on PTK6 signaling will be critical for development of therapeutic strategies that target PTK6 in cancer.

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.