Identification And Characterization Of Mediators Of Toxicity Of Aβ Oligomers By Genome Wide Screening In <i>Caenorhabditis Elegans</i>

Soluble oligomers of the amyloid-β (Aβ) protein play a key role in the pathogenesis of Alzheimer’s disease (AD), although the underlying molecular mechanisms are poorly understood. In order to search for proteins involved in the formation and/or toxicity of Aβ oligomers, a transgenic C. elegans model of AD was used in which inducible expression of Aβ oligomers results in a complete paralysis; in these worms a genetic screen following chemical mutagenesis was applied to discover the genes involved in the Aβ-dependent paralysis (forward genetics). This analysis allowed identification of a mutated clone showing a complete lack of paralysis, despite this it not bear mutations in the Aβ coding region, and accumulates Aβ transcript and protein levels comparable to that of the non-mutated strain. This is the first in vivo model in which the expression of Aβ oligomers do not result in any toxic effect. The genome of the mutated worm was then sequenced and compared with that of the control strain to search for altered genes. Two genes, with no known function, were found to bear a stop codon mutation, likely resulting in the translation of an inactive protein. The rest of the mutations were missense mutations. Among them, point mutations were observed in some genes previously correlated with nematode lifespan and ageing. In C. elegans these biological processes are coordinated by the insulin/IGF-1-like signalling (IIS) pathway, which also regulates the response of the organism to toxic aggregated proteins. Thus, the activation of the IIS pathway was investigated in control and mutated worms. As expected, Aβ expression induced the up-regulation of two genes coding for small heat shock proteins (a class of chaperons known to be involved in AD) in the control strain, whereas these genes were actually down-regulated in the mutated strain. Since heat shock proteins are known to bind Aβ oligomers, these chaperons could directly mediate the formation of toxic amyloid species. Moreover, the results of the whole genome sequencing indicate that several proteins could act as potential novel mediators of Aβ toxicity and could open up new insights for research on age-related, neurodegenerative diseases