Metabolic study of Alzheimer’s disease using mutant C. elegans

Abstract

Alzheimer's disease (AD) is associated with chronic neurodegeneration and is the cause of the most common form of dementia. The main hallmarks of AD are aggregates of amyloid beta (Ab) and formation of hyperphosphorylated tau neurofibrillary tangles (NFTs).nUnfortunately, after more than 100 years of research the exact cause(s) and mechanisms involved in AD progression remain unclarified. Evidence indicates that impaired mitochondrial bioenergetics may precede by decades, the neurodegeneration and cognitive decline associated with AD. Moreover, a genetic association of the core metabolic enzyme dihydrolipoamide dehydrogenase (DLD) with late-onset AD and reduced activity of DLD-containing enzymes suggests glucose hypo-metabolism as a possible cause of AD. Contrary to this, improvements of AD symptoms under caloric restriction or other means of reducing-glucose dependent energy metabolism supports an alternative hypothesis that a decrease in glucose metabolism may be protective. In the present study, we used mutant Caenorhabditis elegans (C. elegans) to investigate the effect of glucose metabolism on AD progression. We initially looked at the role of suppressed DLD-1, and then we focused on the effect of high glucose in AD pathogenesis.Transgenic expression of Ab in C. elegans causes both phenotypic and behavioral defects and results in accumulation of toxic Ab oligomers, culminating in protein aggregation as is normally associated with AD pathophysiology. Ab expression in worm muscle causes agedependent progressive paralysis and impaired acetylcholine neurotransmission while neuronal Ab expression results in defective chemotaxis and impaired serotonin sensitivity as well as reduced fecundity and egg hatching. Suppression of the dld-1 gene alleviated paralysis, improved acetylcholine neurotransmission, enhanced chemotaxis and restored normal sensitivity to serotonin. dld-1 gene suppression also protects vitality as indicated by improved fecundity and egg hatching. Interestingly, protective effects of dld-1 suppression could be reversed using the calcium ionophore (CaI), indicating that the protective mechanism involves calcium signaling. Each of these beneficial effects of dld-1 gene suppression could be mimicked using a specific, small molecule inhibitor ofthe DLD-1 enzyme, 5-methoxyindole-2-carboxylic acid (MICA).High glucose levels are associated with the metabolic disorder, diabetes, which is a major risk factor for AD. In fact, it has been suggested that AD is a neural form of diabetes, type 3 diabetes. If true, drugs used to treat diabetes could act as possible treatments for AD. In this study, I investigated the effect of elevated glucose, the glucose metabolism inhibitor, 2-deoxy-d-glucose n(2DOG) as well as the anti-diabetes drugs, metformin and 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR), on AD progression. Elevated glucose in the growth medium induced hyperactivity, which interfered with the paralysis assays, but it was seen to impair cholinergic neurotransmission, egg laying and hatching. In contrast, 2DOG, metformin and AICAR each significantly alleviated the behavioural problems associated with Ab-expression. These results suggest that regulating glucose metabolism and anti-diabetes drugs may be effective in treating AD.Time-dependent accumulation of Ab worsens behavioral defects in AD. Recent strategies to overcome AD pathogenesis have targeted Ab oligomerization. Suppression of the dld-1 gene, as well as treatment with MICA, 2DOG, metformin and AICAR, each decreased the formation of toxic Ab oligomers, whereas glucose enrichment enhanced Ab oligomerization dose-dependently.An alternative strategy to treat AD is to decrease phosphorylation of tau, as hyperphosphorylation leads to the formation of NFTs. Experimental induction of O-GlcNAcylation, either by suppressing the O-GlcNAcase enzyme with Thiamet-G (TMG) or by suppressing the OGlcNAcase gene (oga-1) itself, resulted in the expected decrease in tau phosphorylation. Furthermore, suppression of the O-GlcNAc transferase gene (ogt-1), which is necessary for OGlcNAcylation, caused an increase in tau phosphorylation. Elevated dietary glucose was expected to induce O-GlcNAcylation, leading to the inhibition of tau phosphorylation. Instead, glucose significantly induced tau phosphorylation at critical residues. Moreover, treatment with TMG could not prevent the glucose-mediated increase in tau phosphorylation. Thus, glucose is a risk factor in tau phosphorylation that is independent of GlcNAcylation of tau in this C. elegans model of AD. While this work suggests that enhancing O-GlcNAcylation to prevent tau phosphorylation may be beneficial, care must be taken to avoid elevated glucose levels.Interestingly, both TMG and oga-1 suppression caused a decrease in Ab-mediated symptoms, whereas suppression of ogt-1 had the opposite effect. This strongly implicates OGlcNAcylation in the toxicity of the Ab peptide despite no previously evidence to suggest this. In conclusion, our results highlight the importance of active energy metabolism on AD progression. Suppression of the dld-1 gene, a key factor in energy metabolism, exposure to the glucose analogue 2DOG, and treatment with the diabetes drug, metformin, all protected against Ab- and tau-mediated toxicity in the C. elegans models.