Intraneuronal amyloid beta and reduced brain volume in a novel APP T714I mouse model for Alzheimer's disease.

Transgenic mouse models of Alzheimer's disease (AD) expressing high levels of amyloid precursor protein (APP) with familial AD (FAD) mutations have proven to be extremely useful in understanding pathogenic processes of AD especially those that involve amyloidogenesis. We earlier described Austrian APP T714I pathology that leads to one of the earliest AD age-at-onsets with abundant intracellular and extracellular amyloid deposits in brain. The latter strikingly was non-fibrillar diffuse amyloid, composed of N-truncated A beta 42 in absence of A beta 40. In vitro, this mutation leads to one of the highest A beta 42/A beta 40 ratios among all FAD mutations. We generated an APP T714I transgenic mouse model that despite having 10 times lower transgene than endogenous murine APP deposited intraneuronal A beta in brain by 6 months of age. Accumulations increased with age, and this was paralleled by decreased brain sizes on volumetric MRI, compared to age-matched and similar transgene-expressing APP wild-type mice, although, with these levels of transgenic expression we did not detect neuronal loss or significant memory impairment. Immunohistochemical studies revealed that the majority of the intraneuronal A beta deposits colocalized with late endosomal markers, although some A beta inclusions were also positive for lysosomal and Golgi markers. These data support earlier observations of A beta accumulation in the endosomal-lysosomal pathway and the hypothesis that intraneuronal accumulation of A beta could be an important factor in the AD pathogenesis.

Ataxin-7 aggregation and ubiquitination in infantile SCA7 with 180 CAG repeats.

Extremely long (>150) CAG repeats are often used to create models of polyglutamine diseases yet are very rare in humans where they manifest as pediatric multisystem syndromes of little specificity. Here, we describe an infant with 180 CAG repeats in the spinocerebellar ataxia type 7 gene and focus on systemic ataxin-7 aggregation. This was found in many organs, including the cardiovascular system. In the brain, the hippocampus emerged as a principal site of ataxin-7 aggregation without cell loss. We note differential ubiquitination of aggregates and discuss how this may relate to selective vulnerability.

Proteasome degrades soluble expanded polyglutamine completely and efficiently.

To date, nine progressive neurodegenerative diseases are caused by expansion of the CAG repeat coding for polyglutamine, including Huntington's disease and several forms of spinocerebellar ataxia. Expanded polyglutamine causes dominant toxic gain-of-function related to its ability to aggregate. Polyglutamine aggregates inhibit the proteasome, suggesting that reduced degradation of misfolded proteins might contribute to polyglutamine toxicity. Moreover, several observations indicate that soluble proteins harboring expanded polyglutamine display altered turnover. To examine whether soluble polyglutamine interfered with proteasome-mediated degradation, we analyzed degradation of model proteasome substrates carrying either 103 or 25 glutamines in transfected cells. Expanded and normal size polyglutamine were degraded to completion and with similar efficiency. Moreover, targeting of expanded polyglutamine for proteasome-mediated degradation did not compromise proteasome activity. Thus, we propose that polyglutamine-containing disease proteins can be readily digested by the proteasome if they carried a degradation signal.

Hot-spot residue in small heat-shock protein 22 causes distal motor neuropathy.

Distal hereditary motor neuropathies are pure motor disorders of the peripheral nervous system resulting in severe atrophy and wasting of distal limb muscles. In two pedigrees with distal hereditary motor neuropathy type II linked to chromosome 12q24.3, we identified the same mutation (K141N) in small heat-shock 22-kDa protein 8 (encoded by HSPB8; also called HSP22). We found a second mutation (K141E) in two smaller families. Both mutations target the same amino acid, which is essential to the structural and functional integrity of the small heat-shock protein alphaA-crystallin. This positively charged residue, when mutated in other small heat-shock proteins, results in various human disorders. Coimmunoprecipitation experiments showed greater binding of both HSPB8 mutants to the interacting partner HSPB1. Expression of mutant HSPB8 in cultured cells promoted formation of intracellular aggregates. Our findings provide further evidence that mutations in heat-shock proteins have an important role in neurodegenerative disorders.

Pathogenesis of polyglutamine disorders: aggregation revisited.

Expansion of CAG trinucleotide repeats coding for polyglutamine in unrelated proteins causes at least nine late-onset progressive neurodegenerative disorders, including Huntington's disease and a number of spinocerebellar ataxias. Expanded polyglutamine provokes a dominant gain-of-function neurotoxicity, regardless of the specific protein context within which it resides. Nevertheless, the protein context does modulate polyglutamine toxicity, as evidenced by the distinct clinical and pathological features of the various disorders. Importantly, polyglutamine toxicity might derive from its ability to aggregate. Indeed, aggregation probably underlies some defining attributes of the polyglutamine disorders, such as their late onset, progressive nature, and the dependence of onset age on polyglutamine length. However, the central role of aggregation in polyglutamine pathogenesis has been challenged by several studies, which instead argued that the soluble form of the disease proteins is responsible for neuronal damage. Thus, the question whether polyglutamine aggregates are deleterious, harmless or protective remains the most passionately disputed issue in the study of these diseases. In this review, we attempt to reconcile some of these controversies.