Molecular Studies On The Inhibition Of Polyglutamine-mediated Proteotoxicity By Astragalus Membranaceus Polysaccharide

Late-onset neurodegenerative diseases are characterized by progressive accumulation of aggregation-prone proteins and global disruption of the proteostasis network, e.g. abnormal polyglutamine (polyQ) aggregation in Huntington's disease. Astragalus membranaceus polysaccharides have recently been shown to modulate aging and proteotoxic stress pathways. Using mammalian cells and Caenorhabditis elegans models, we now show that astragalan, an acidic A. membranaceus polysaccharide consisting of 19.5% arabinose,6.4% rhamnose,15.6% galactose, 29.5% galacturonic acid and 29.0% glucose, not only reduces polyQ aggregation but also alleviates the associated neurotoxicity, and that this polyQ-inhibiting activity is not dependent on autophagic pathways. We also reveal that astragalan can extend the adult lifespan of wild-type and polyQ nematodes, indicating a connection of its anti-aging benefit with the toxicity-suppressing effect. Further examination demonstrates that astragalan can extend the lifespan of daf-2 and age-1, but not daf-16, mutant nematodes of the insulin-like aging and stress pathway, suggesting a lifespan-regulation signaling independent of DAF-2/IGF-1R but dependent on DAF-16/FOXO transcription factor, a pivotal integrator of divergent signaling pathways related to both lifespan regulation and stress resistance. From cDNA microarray experiments using wild-type nematodes we show that 223 genes are up-regulated and 176 genes down-regulated for >2 folds after astragalan treatment, including genes related to aging, proteostasis and stress regulation. We also demonstrate by real-time PCR that a subset of DAF-16 downstream genes are regulated by astragalan. including the DAF-16 transcriptional target gene scl-20, which is itself constitutively up-regulated in transgenic polyQ nematodes. Interestingly, although scl-20 is up-regulated by astragalan in wild-type and polyQ nematodes but not in daf-16 mutant, further suggesting the dependence of astragalan on DAF-16 transcription factor. These findings not only provide a revealing insight into the potential of stress and lifespan regulators in the prevention of proteotoxic disorders but also provide an indispensable basis for the development of neurodegenerative glycotherapeutics from astragalan and other polysaccharides.