A Caenorhabditis elegans model system for amylopathy study

Invited review: Unearthing the mechanisms of age-related neurodegenerative disease using Caenorhabditis elegans

As human life expectancy increases, neurodegenerative diseases present a growing public health threat, for which there are currently few effective treatments. There is an urgent need to understand the molecular and genetic underpinnings of these disorders so new therapeutic targets can be identified. Here we present the argument that the simple nematode worm Caenorhabditis elegans is a powerful tool to rapidly study neurodegenerative disorders due to their short lifespan and vast array of genetic tools, which can be combined with characterization of conserved neuronal processes and behavior orthologous to those disrupted in human disease. We review how pre-existing C. elegans models provide insight into human neurological disease as well as an overview of current tools available to study neurodegenerative diseases in the worm, with an emphasis on genetics and behavior. We also discuss open questions that C. elegans may be particularly well suited for in future studies and how worms will be a valuable preclinical model to better understand these devastating neurological disorders.

In vivo induction of membrane damage by β-amyloid peptide oligomers

Exposure to the β-amyloid peptide (Aβ) is toxic to neurons and other cell types, but the mechanism(s) involved are still unresolved. Synthetic Aβ oligomers can induce ion-permeable pores in synthetic membranes, but whether this ability to damage membranes plays a role in the ability of Aβ oligomers to induce tau hyperphosphorylation, or other disease-relevant pathological changes, is unclear. To examine the cellular responses to Aβ exposure independent of possible receptor interactions, we have developed an in vivo C. elegans model that allows us to visualize these cellular responses in living animals. We find that feeding C. elegans E. coli expressing human Aβ induces a membrane repair response similar to that induced by exposure to the CRY5B, a known pore-forming toxin produced by B. thuringensis. This repair response does not occur when C. elegans is exposed to an Aβ Gly37Leu variant, which we have previously shown to be incapable of inducing tau phosphorylation in hippocampal neurons. The repair response is also blocked by loss of calpain function, and is altered by loss-of-function mutations in the C. elegans orthologs of BIN1 and PICALM, well-established risk genes for late onset Alzheimer's disease. To investigate the role of membrane repair on tau phosphorylation directly, we exposed hippocampal neurons to streptolysin O (SLO), a pore-forming toxin that induces a well-characterized membrane repair response. We find that SLO induces tau hyperphosphorylation, which is blocked by calpain inhibition. Finally, we use a novel biarsenical dye-tagging approach to show that the Gly37Leu substitution interferes with Aβ multimerization and thus the formation of potentially pore-forming oligomers. We propose that Aβ-induced tau hyperphosphorylation may be a downstream consequence of induction of a membrane repair process.

Genetic and Pharmacological Discovery for Alzheimer's Disease Using Caenorhabditis elegans

The societal burden presented by Alzheimer's disease warrants both innovative and expedient means by which its underlying molecular causes can be both identified and mechanistically exploited to discern novel therapeutic targets and strategies. The conserved characteristics, defined neuroanatomy, and advanced technological application of Caenorhabditis elegans render this metazoan an unmatched tool for probing neurotoxic factors. In addition, its short lifespan and importance in the field of aging make it an ideal organism for modeling age-related neurodegenerative disease. As such, this nematode system has demonstrated its value in predicting functional modifiers of human neurodegenerative disorders. Here, we review how C. elegans has been utilized to model Alzheimer's disease. Specifically, we present how the causative neurotoxic peptides, amyloid-β and tau, contribute to disease-like neurodegeneration in C. elegans and how they translate to human disease. Furthermore, we describe how a variety of transgenic animal strains, each with distinct utility, have been used to identify both genetic and pharmacological modifiers of toxicity in C. elegans. As technological advances improve the prospects for intervention, the rapidity, unparalleled accuracy, and scale that C. elegans offers researchers for defining functional modifiers of neurodegeneration should speed the discovery of improved therapies for Alzheimer's disease.

Guanine nucleotide exchange factor OSG-1 confers functional aging via dysregulated Rho signaling in Caenorhabditis elegans neurons

Rho signaling regulates a variety of biological processes, but whether it is implicated in aging remains an open question. Here we show that a guanine nucleotide exchange factor of the Dbl family, OSG-1, confers functional aging by dysregulating Rho GTPases activities in C. elegans. Thus, gene reporter analysis revealed widespread OSG-1 expression in muscle and neurons. Loss of OSG-1 gene function was not associated with developmental defects. In contrast, suppression of OSG-1 lessened loss of function (chemotaxis) in ASE sensory neurons subjected to conditions of oxidative stress generated during natural aging, by oxidative challenges, or by genetic mutations. RNAi analysis showed that OSG-1 was specific toward activation of RHO-1 GTPase signaling. RNAi further implicated actin-binding proteins ARX-3 and ARX-5, thus the actin cytoskeleton, as one of the targets of OSG-1/RHO-1 signaling. Taken together these data suggest that OSG-1 is recruited under conditions of oxidative stress, a hallmark of aging, and contributes to promote loss of neuronal function by affecting the actin cytoskeleton via altered RHO-1 activity.