Studies of the mutant gene in Huntington's disease, and for eight related neurodegenerative disorders, have identified polyglutamine (polyQ) expansions as a basis for cellular toxicity. This finding has led to a disease hypothesis that protein aggregation and cellular dysfunction can occur at a threshold of approximately 40 glutamine residues. Here, we test this hypothesis by expression of fluorescently tagged polyQ proteins (Q29, Q33, Q35, Q40, and Q44) in the body wall muscle cells of Caenorhabditis elegans and show that young adults exhibit a sharp boundary at 35-40 glutamines associated with the appearance of protein aggregates and loss of motility. Surprisingly, genetically identical animals expressing near-threshold polyQ repeats exhibited a high degree of variation in the appearance of protein aggregates and cellular toxicity that was dependent on repeat length and exacerbated during aging. The role of genetically determined aging pathways in the progression of age-dependent polyQ-mediated aggregation and cellular toxicity was tested by expressing Q82 in the background of age-1 mutant animals that exhibit an extended lifespan. We observed a dramatic delay of polyQ toxicity and appearance of protein aggregates. These data provide experimental support for the threshold hypothesis of polyQ-mediated toxicity in an experimental organism and emphasize the importance of the threshold as a point at which genetic modifiers and aging influence biochemical environment and protein homeostasis in the cell.T he folding and maintenance of proteins in their native conformation is essential to cellular function. Disruption of protein-folding homeostasis, leading to the appearance of protein aggregates, is associated with an increasing number of human diseases (1, 2). A prototypical class of these disorders, composed of at least eight progressive neurodegenerative diseases including Huntington's disease, is associated with genes containing (CAG) n trinucleotide repeats encoding polyglutamine (polyQ) tracts in otherwise unrelated proteins (3, 4). Expression of expanded polyQ, with or without flanking sequences, is sufficient to recapitulate the pathological features of the diseases in multiple model systems, supporting a central role for the expansion in the etiology of these disorders (5-7).Molecular genetic studies have established that huntingtin alleles from normal chromosomes contain fewer than 30-34 CAG repeats, whereas those from affected chromosomes contain more than 35-40 repeats (8, 9). These observations have led to the suggestion of a 35-40-residue threshold at which the disease gene products are converted to a proteotoxic state. Analysis of patient databases has established a strong inverse correlation between repeat length and age of onset (9-11). However, both this correlation and disease penetrance are much weaker for repeats of 42 or fewer residues (12), suggesting that substantial variation in the behavior of polyQ-containing proteins can exist at near-threshold repeat lengths, which influences the course ...

Numerous human diseases are associated with the chronic expression of misfolded and aggregation-prone proteins. The expansion of polyglutamine residues in unrelated proteins is associated with the early onset of neurodegenerative disease. To understand how the presence of misfolded proteins leads to cellular dysfunction, we employed Caenorhabditis elegans polyglutamine aggregation models. Here, we find that polyglutamine expansions disrupted the global balance of protein folding quality control, resulting in the loss of function of diverse metastable proteins with destabilizing temperature-sensitive mutations. In turn, these proteins, although innocuous under normal physiological conditions, enhanced the aggregation of polyglutamine proteins. Thus, weak folding mutations throughout the genome can function as modifiers of polyglutamine phenotypes and toxicity.

The basis of neuron-specific pathogenesis, resulting from the expression of misfolded proteins, is poorly understood and of central importance to an understanding of the cell-type specificity of neurodegenerative disease. In this study, we developed a new model for neuron-specific polyQ pathogenesis in Caenorhabditis elegans by pan-neuronal expression that exhibits polyQ length-dependent aggregation, neurotoxicity, and a pathogenic threshold at a length of 35-40 glutamines. Analysis of specific neurons in C. elegans revealed that only at the threshold length, but not at shorter or longer lengths, polyQ proteins can exist in a soluble state in certain lateral neurons or in an aggregated state in motor neurons of the same animal. These results provide direct experimental evidence that the expression of a single species of a toxic misfolded protein can exhibit a range of neuronal consequences.