Showing papers on "Proteotoxicity published in 2002"

The threshold for polyglutamine-expansion protein aggregation and cellular toxicity is dynamic and influenced by aging in Caenorhabditis elegans

Abstract: 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.

Proteotoxicity in the endoplasmic reticulum: lessons from the Akita diabetic mouse

Abstract: Mutations that impair polypeptide folding commonly result in an unstable and hypofunctional gene product. Their phenotype reflects this loss of function, and the resulting genetic disorder is usually transmitted as a recessive trait. Common forms of cystic fibrosis, hemophilia, and familial hypercholesterolemia are examples of this genetic mechanism in action. A second class of mutations (so-called gain-of-function mutations) encodes proteins with new functions, whose associated disorders are typically transmitted as dominant traits. Some of these are “dominant negative” alleles whose encoded protein can interact with components of the cellular machinery normally accessed by the wild-type gene product. With varying degrees of specificity, these mutations affect the activities of the wild-type allele, for example by disrupting the assembly of a multisubunit complex. Recent observations suggest that gain-of-function mutations that affect protein folding can also impair cellular function by less specific mechanisms related to the ability of the mutant protein to challenge the folding capacity in specific cellular compartments. Such mutant proteins are hypothesized to act as proteotoxins and may play a role in important human diseases (1, 2). The paper by Oyadomari et al. (3) appearing in this issue of the JCI addresses important issues related to proteotoxicity in the endoplasmic reticulum (ER).

Stressful preconditioning and HSP70 overexpression attenuate proteotoxicity of cellular ATP depletion

Abstract: Rat H9c2 myoblasts were preconditioned by heat or metabolic stress followed by recovery under normal conditions. Cells were then subjected to severe ATP depletion, and stress-associated proteotoxicity was assessed on 1) the increase in a Triton X-100-insoluble component of total cellular protein and 2) the rate of inactivation and insolubilization of transfected luciferase with cytoplasmic or nuclear localization. Both heat and metabolic preconditioning elevated the intracellular heat shock protein 70 (HSP70) level and reduced cell death after sustained ATP depletion without affecting the rate and extent of ATP decrease. Each preconditioning attenuated the stress-induced insolubility among total cellular protein as well as the inactivation and insolubilization of cytoplasmic and nuclear luciferase. Transient overexpression of human HSP70 in cells also attenuated both the cytotoxic and proteotoxic effects of ATP depletion. Quercetin, a blocker of stress-responsive HSP expression, abolished the effects of stressful preconditioning but did not influence the effects of overexpressed HSP70. Analyses of the cellular fractions revealed that both the stress-preconditioned and HSP70-overexpressing cells retain the soluble pool of HSP70 longer during ATP depletion. Larger amounts of other proteins coimmunoprecipitated with excess HSP70 compared with control cells deprived of ATP. This is the first demonstration of positive correlation between chaperone activity within cells and their viability in the context of ischemia-like stress.

Transmission of proteotoxicity across cellular compartments

Abstract: An important subgroup of human neurodegenerative diseases is associated with abnormal expansions of glutamine repeats found in several otherwise unrelated proteins (Zoghbi and Orr 2000). Interest in these polyglutamine diseases is fueled both by their clinical significance and by the belief that lessons gleaned from these relatively rare conditions will apply to other more prevalent human neurodegenerative disorders and perhaps more generally to other diseases of aging. The basis for this belief is the observation that polyglutamine diseases and common neurodegenerative disorders, such as Alzheimer’s disease and Parkinson’s disease, share as a common feature the accumulation of abnormal protein aggregates in and around affected neurons (Kaytor and Warren 1999). In the polyglutamine diseases there is a good correlation between the length of the glutamine repeat, the tendency of the affected protein to assume abnormal aggregation-prone states of folding, and the occurrence of neurodegeneration (Gusella and MacDonald 2000; Orr 2001). Furthermore, there is reason to believe that despite the dissimilarities in structure and function of the proteins that “host” the glutamine repeat, once the polyglutamine expansion has reached a critical length, it imposes a common abnormal folding state on the affected protein (Perutz 1996). Together with the dominant inheritance pattern of the associated diseases, these observations suggest that the abnormal polyglutamine repeat converts host proteins into toxic entities, or proteotoxins (Hightower 1991). Attempts to understand the pathophysiology of polyglutamine diseases have therefore focused on the physical state of the abnormal protein, the cellular compartment in which it is found, and the impact of the abnormal protein on cell physiology. A paper in this issue of Genes & Development reports on a pathogenic polyglutamine protein that accumulates as a nuclear aggregate yet triggers the unfolded protein response—a cellular response to unfolded and misfolded proteins in the endoplasmic reticulum (Nishitoh et al. 2002). We will discuss some of the implications of this finding whereby the impact of a polyglutamine-containing proteotoxin can be transmitted from one cellular compartment to another.