Watching worms whither: modeling neurodegeneration in C. elegans

Abstract

Caenorhabditis elegans is increasingly being used to study neurodegenerative diseases. Nematodes are translucent, which facilitates study of particular neurons in the living animal, and easy to manipulate genetically. Despite vast evolutionary divergence, human proteins are functionally active when expressed in C. elegans, and disease-linked mutations in these proteins also cause phenotypic changes in the nematode. In this chapter, we review use of C. elegans to investigate the pathophysiology of Alzheimer's disease, Parkinson's disease, and axonal degeneration. Studies of presenilin, β-amyloid, tau, α-synuclein, and LRRK2 all produce strong phenotypic effects in C. elegans, and in many cases reproduce selective neuronal vulnerability observed in humans. Disease-linked mutations enhance degeneration in the C. elegans models. These studies are increasingly leading to high-throughput screens that identify novel genes and novel pharmaceuticals that modify the disease course.

Figure 1

A. C. elegans life cycle. C. elegans progress through four larval stages until it reaches adulthood, whereupon it lays eggs. Larval development takes 2–3 days, depending on the temperature at which the nematode is grown. Adult nematodes can live for 2–3 weeks. Nematodes that encounter food deprivation during early development enter an alternate stage, termed dauer. The dauers can survive for extended periods of time on very little food, and don’t continue through development until nutrient resources are restored. B. Neurons in the nematode are mainly present in a region in the head termed the nerve ring. Panel B shows the head region in a line of C. elegans the human protein, Tar DNA Binding Protein – 43 (TDP-43, wild type) linked to GFP driven by the neuronal selective synaptobrevin promoter. TDP-43 localizes exclusively to neuronal nuclei. The mouth and pharynx are observed in the background, with the pharynx surrounded by GFP-positive neurons.

Figure 2

Localization of dopaminergic neurons in C. elegans shown using GFP driven by the dopamine transporter promoter. C. elegans has 6 dopaminergic neurons in the head; 4 neurons are termed CEP and 2 are termed ADE (arrows).

Figure 3

Fluorescence of dopaminergic neurons following treatment with three different RNAi constructs (empty vector control, RNAi #4 and RNAi #16), followed by exposure to 6-hydroxydopamine (50 mM) and subsequent aging for 4 more days with continued exposure to RNAi. RNAi #4 sensitized the dopaminergic neuronsto 6-hydroxydopamine, while RNAi #16 protected the dopaminergic neurons against 6-hydroxydopamine.

Figure 4

In vivo neuronal regeneration in C. elegans after femtosecond laser surgery. Top: An intact mechanosensory neuron imaged in vivo using transgenically expressed fluorescent markers. Middle: Immediately following laser exposure, a clean break is seen at the target point (arrow). Femtosecond laser surgery allows in vivo ablation on the submicron scale and snipping of single nerve fibers within an intact adult C. elegans. Bottom: 24 h after laser surgery substantial regenerative growth is observed. In this example growth is seen both at the original cut point as well as sprouting directly from the cell soma.