Amyloid Formation by Mutant Huntingtin: Threshold, Progressivity and Recruitment of Normal Polyglutamine Proteins

Abstract

Huntington's disease (HD) is caused by an expanded CAG trinucleotide repeat encoding a tract of consecutive glutamines near the amino terminus of huntingtin, a large protein of unknown function. It has been proposed that the expanded polyglutamine stretch confers a new property on huntingtin and thereby causes cell and region-specific neurodegeneration. Genotype-phenotype correlations predict that this novel property appears above a threshold length (approximately 38 glutamines), becomes progressively more evident with increasing polyglutamine length, is completely dominant over normal huntingtin and is not appreciably worsened by a double genetic dose in HD homozygotes. Recently, an amino terminal fragment of mutant huntingtin has been found to form self-initiated fibrillar aggregates in vitro. We have tested the capacity for aggregation to assess whether this property matches the criteria expected for a fundamental role in HD pathogenesis. We find that that in vitro aggregation displays a threshold and progressivity for polyglutamine length remarkably similar to the HD disease process. Moreover, the mutant huntingtin amino terminus is capable of recruiting into aggregates normal glutamine tract proteins, such as the amino terminal segments of both normal huntingtin and of TATA-binding protein (TBP). Our examination of in vivo aggregates from HD post-mortem brains indicates that they contain an amino terminal segment of huntingtin of between 179 and 595 residues. They also contain non-huntingtin protein, as evidenced by immunostaining for TBP. Interestingly, like the in vitro aggregates, aggregates from HD brain display Congo red staining with green birefringence characteristic of amyloid. Our data support the view that the expanded polyglutamine segment confers on huntingtin a new property that plays a determining role in HD pathogenesis and could be a target for treatment. Moreover, the new property might have its toxic consequences by interaction with one or more normal polyglutamine-containing proteins essential for the survival of target neurons.

Increased susceptibility of cytoplasmic over nuclear polyglutamine aggregates to autophagic degradation

“…The length of polyQ tracts in normal humans is polymorphic but always below a threshold of 35-40. PolyQ repeats above this threshold are invariably associated with disease, with a strong inverse correlation between repeat length and age-of-onset of neurological disease (2). These findings, together with studies on Htt fragments expressed in cell culture (3) or polyQ peptides in vitro (4), support a model in which glutamine repeats above the threshold adopt a nonnative conformation that is highly prone to self-associate into high-molecular-weight, stable aggregates.…”

Polyglutamine fibrillogenesis: The pathway unfolds

“…N ine neurodegenerative diseases are caused by expanding CAG repeats coding for polyglutamine (polyGln) (1)(2)(3)(4). These include Huntington's disease, dentatorubral and pallidoluysian atrophy, several forms of spino-cerebellar ataxia, and spinal and bulbar muscular atrophy.…”

Spontaneous self-assembly of pathogenic huntingtin exon 1 protein into amyloid structures

“…This suggests that the nuclear environment is especially vulnerable to polyQ protein aggregates. This could be due to the fact that polyQ aggregates recruit other polyQ or Q-rich proteins such as the TATA-binding protein (TBP), which subsequently lose their normal cellular functions [30]. Finally, evidence demonstrates that in vitro-produced fibrillar HTTex1 aggregates, rather than oligomers or monomers, are toxic for mammalian cells [15].…”