Huntington disease (HD) is a progressive neurodegenerative disorder caused by expression of polyglutamine-expanded mutant huntingtin protein (mhtt). Most evidence indicates that soluble mhtt species, rather than insoluble aggregates, are the important mediators of HD pathogenesis. However, the differential roles of soluble monomeric and oligomeric mhtt species in HD and the mechanisms of oligomer formation are not yet understood. We have shown previously that copper interacts with and oxidizes the polyglutamine-containing N171 fragment of huntingtin. In this study we report that oxidation-dependent oligomers of huntingtin form spontaneously in cell and mouse HD models. Levels of these species are modulated by copper, hydrogen peroxide, and glutathione. Mutagenesis of all cysteine residues within N171 blocks the formation of these oligomers. In cells, levels of oligomerization-blocked mutant N171 were decreased compared with native N171. We further show that a subset of the oligomerization-blocked form of glutamine-expanded N171 huntingtin is rapidly depleted from the soluble pool compared with "native " mutant N171. Taken together, our data indicate that huntingtin is subject to specific oxidations that are involved in the formation of stable oligomers and that also delay removal from the soluble pool. These findings show that inhibiting formation of oxidation-dependent huntingtin oligomers, or promoting their dissolution, may have protective effects in HD by decreasing the burden of soluble mutant huntingtin. Huntington disease (HD)2 is a progressive neurodegenerative disorder caused by a glutamine-encoding CAG expansion in the huntingtin gene (1). Mutant huntingtin protein (mhtt) is cleaved into a number of polyglutamine containing N-terminal fragments that misfold and form soluble monomeric and oligomeric proteins as well as insoluble aggregates. Several publications have shown that soluble N-terminal fragments of mhtt are important toxic factors that drive HD (2-4). Further, in HD and other neurodegenerative diseases, there is evidence that soluble oligomers may be particularly important (2,(5)(6)(7)(8).Soluble oligomers of mhtt may form or be stabilized through a number of mechanisms including noncovalent polyglutamine-dependent interactions (5) and possibly transglutaminase-mediated covalent cross-links (9). In addition, oxidation of huntingtin could also promote oligomerization (10). In general, cysteine oxidation is an important mechanism for forming intermolecular cross-links that may stabilize protein oligomers (11, 12). Site-specific cysteine oxidation may regulate protein function by affecting structure including oligomerization state (13-15). There is growing evidence that a variety of protein residues up and downstream of the mhtt polyglutamine tract impact HD progression through providing sites of post-translational modification that affect huntingtin degradation, cleavage, or cell location (16 -19). The N terminus of huntingtin contains several cysteine residues, the most C-terminal of which are ...

Alzheimer's disease is characterized by accumulation of toxic β-amyloid (Aβ) in the brain and neuronal death. Several mutations in presenilin (PS1) and β-amyloid precursor protein (APP) associate with an increased Aβ42/40 ratio. Aβ42, a highly fibrillogenic species, is believed to drive Aβ aggregation. Factors shifting γ-secretase cleavage of APP to produce Aβ42 are unclear. We investigate the molecular mechanism underlying altered Aβ42/40 ratios associated with APP mutations at codon 716 and 717. Using FRET-based fluorescence lifetime imaging to monitor APP-PS1 interactions, we show that I716F and V717I APP mutations increase the proportion of interacting molecules earlier in the secretory pathway, resulting in an increase in Aβ generation. A PS1 conformation assay reveals that, in the presence of mutant APP, PS1 adopts a conformation reminiscent of FAD-associated PS1 mutations, thus influencing APP binding to PS1/γ-secretase. Mutant APP affects both intracellular location and efficiency of APP-PS1 interactions, thereby changing the Aβ42/40 ratio.

In neurons, Presenilin 1(PS1)/γ‐secretase is located at the synapses, bound to N‐cadherin. We have previously reported that N‐cadherin‐mediated cell–cell contact promotes cell‐surface expression of PS1/γ‐secretase. We postulated that N‐cadherin‐mediated trafficking of PS1 might impact synaptic PS1‐amyloid precursor protein interactions and Aβ generation. In the present report, we evaluate the effect of N‐cadherin‐based contacts on Aβ production. We demonstrate that stable expression of N‐cadherin in Chinese hamster ovary cells, expressing the Swedish mutant of human amyloid precursor protein leads to enhanced secretion of Aβ in the medium. Moreover, N‐cadherin expression decreased Aβ42/40 ratio. The effect of N‐cadherin expression on Aβ production was accompanied by the enhanced accessibility of PS1/γ‐secretase to amyloid precursor protein as well as a conformational change of PS1, as demonstrated by the fluorescence lifetime imaging technique. These results indicate that N‐cadherin‐mediated synaptic adhesion may modulate Aβ secretion as well as the Aβ42/40 ratio via PS1/N‐cadherin interactions.