In AD, Aβ forms amyloid fibrils, which accumulate in deposits known as plaques. To study aggregation of Aβ, the human Aβ peptide was overexpressed in C. elegans. By using the C. elegans Myosin/unc-54 muscle-specific promoter to drive expression, robust expression levels were obtained. A 189-base pair fragment was overexpressed coding for amino acids 1–42 of Aβ fused to a synthetic signal peptide driven by the unc-54 promoter. Transgenic worms expressing this transgene showed formation of aggregates with important characteristic of amyloid fibrils, as shown by Thioflavin S binding, an amyloid-specific dye, which has altered fluorescent properties on binding. In addition, paralysis of the nematodes, which can be easily distinguished by dissection microscopy, occurred indicating a specific toxicity of Aβ to the muscle cells (25). Interestingly, oligomeric species of Aβ were detected in these strains that might be similar to the neurotoxic Aβ-derived diffusible ligand (ADDL) (24).

The insulin/insulin growth factor (IGF)-1 like signaling (IIS) pathway strongly affects lifespan and stress resistance in C. elegans. Inhibition of daf-2/IGF-1 receptor by RNAi increases lifespan and stress resistance in nematodes. Mice deficient in the IGF-1 receptor also show increased lifespan, indicating the conservation of this mechanism (26). The effect of daf-2 partially depends on heat shock factor-1 (hsf-1), an important transcriptional regulator of heat shock genes. Interestingly, knockdown of the daf-2 gene in Aβ transgenic nematodes results in decreased paralysis, whereas hsf-1 knockdown increases paralysis, which is accompanied by alterations in the aggregation phenotype (27). It will be interesting to find out if these genetic pathways also affect Aβ aggregation and toxicity in mice and humans, which would directly link aging with the prevalence of disease related to Aβ aggregation.

Another protein that is associated with forms of dementia is the presynaptic protein α-synuclein. This protein provides a remarkable link between several neurodegenerative diseases, most of which involve dementia. It is the main constituent of protein inclusions in the brains of PD patients that represent the pathological hallmark of this disease. Such inclusions (known as Lewy bodies) also occur in a subtype of dementia called DLB, which is currently the second most prevalent cause of dementia. Importantly, DLB often has an overlap with Alzheimer’s pathology, and is sometimes difficult to distinguish from either AD or dementia related to PD.

Interestingly, α-synuclein mutations and multiplications of the α-synuclein locus have been found to cause familial forms of PD. One familial PD mutation, E46K, is also known to be associated with dementia (28). Some aspect of aggregation of α-synuclein is most likely directly involved in toxicity with the cells that degenerate in these diseases. The current hypothesis is that multimers (multiple associated molecules) of α-synuclein are somehow able to damage cells leading to degeneration. Although it is very likely that α-synuclein is directly involved in the pathogenesis, the mode of toxicity remains unknown. Physiologically, α-synuclein appears to regulate docking events of synaptic vesicles, likely by influencing the fusion of membranes (28–32).

To study expression and accumulation of a disease-related protein such as α-synuclein in a living animal, one can make the accumulation visible under a fluorescence microscope by fusion to a fluorescent molecule such as Yellow Fluorescent Protein (YFP). The C. elegans body wall muscle provides a system in which one can track accumulation of various disease-related aggregating proteins during aging (33,34). The muscle cells are easily seen under a microscope, and are amenable to RNAi by feeding (Fig. 1). In addition, the robust expression levels obtained by the muscle-specific unc-54 promoter yield high protein levels, which facilitate biochemical and cell biological analysis of protein expression.

Transgenic strains expressing human α-synuclein fused to YFP in the body wall muscle show accumulation of the fusion protein during aging (35). Interestingly, the inclusions formed are mobile at young age, but show properties of immobilized aggregated protein at old age. To find genes involved in the formation of α-synuclein inclusion, a genome-wide screen was performed for genes that result in increased aggregation when knocked down by RNAi. Of the 80 genes found, genes that function in endomembrane-related compartments of the cell are overrepresented, which indicates a role for the endomembrane system in age-related synucleinopathies such as DLB (Fig. 2).

Tau (MAPT) is a microtubule-associated protein (MAP) mainly found in neuronal axons where it binds and stabilizes microtubules. In sporadic AD and familial frontotemporal dementia with Parkinsonism-17 (FTDP-17), a form of presenile dementia affecting the frontal and temporal cortex and some subcortical nuclei, hyperphosphorylated tau protein forms deposits in the brain. To model disease related to tau expression and deposition in C. elegans, Kraemer and colleagues have expressed both wild-type and mutated (301L and 337M) tau protein in C. elegans neurons (36). These nematodes developed a progressive phenotype of defective motility known as “uncoordinated phenotype” that can be distinguished by dissection microscopy, which was more apparent in the mutants. Interestingly, another characteristic of disease was captured in this model, since hyperphosphorylation of tau also appeared to occur in the transgenic lines (36). To identify modifiers of this toxicity, a genome-wide RNAi screen for enhancement of tau-related toxicity was performed (37). Some of the genes found are known to be involved in tau-related pathology, such as Glycogen synthase kinase 3β (GSK-3β) that can phosphorylate tau. New components have also been found, such as homologs of carboxyl terminus of Hsp 70-interacting protein (CHIP) and Heat shock cognate 70 (Hsc70) that cooperate in ubiquitination and proteasomal degradation. Thus, such genes might normally be involved in protection against tau-mediated toxicity.

Future work will show what modifier genes found in these genome-wide screens can tell us about the disease mechanism, and whether they represent diagnostic or even therapeutic targets to treat disease.