Excitation-induced ataxin-3 aggregation in neurons from patients with Machado–Joseph disease

Abstract

Machado-Joseph disease (MJD; also called spinocerebellar ataxia type 3) is a dominantly inherited late-onset neurodegenerative disorder caused by expansion of polyglutamine (polyQ)-encoding CAG repeats in the MJD1 gene (also known as ATXN3). Proteolytic liberation of highly aggregation-prone polyQ fragments from the protective sequence of the MJD1 gene product ataxin 3 (ATXN3) has been proposed to trigger the formation of ATXN3-containing aggregates, the neuropathological hallmark of MJD. ATXN3 fragments are detected in brain tissue of MJD patients and transgenic mice expressing mutant human ATXN3(Q71), and their amount increases with disease severity, supporting a relationship between ATXN3 processing and disease progression. The formation of early aggregation intermediates is thought to have a critical role in disease initiation, but the precise pathogenic mechanism operating in MJD has remained elusive. Here we show that L-glutamate-induced excitation of patient-specific induced pluripotent stem cell (iPSC)-derived neurons initiates Ca(2+)-dependent proteolysis of ATXN3 followed by the formation of SDS-insoluble aggregates. This phenotype could be abolished by calpain inhibition, confirming a key role of this protease in ATXN3 aggregation. Aggregate formation was further dependent on functional Na(+) and K(+) channels as well as ionotropic and voltage-gated Ca(2+) channels, and was not observed in iPSCs, fibroblasts or glia, thereby providing an explanation for the neuron-specific phenotype of this disease. Our data illustrate that iPSCs enable the study of aberrant protein processing associated with late-onset neurodegenerative disorders in patient-specific neurons.

The Role for Alterations in Neuronal Activity in the Pathogenesis of Polyglutamine Repeat Disorders

“…The formation of early aggregation intermediates is suggested to play an important role in polyQ diseases [84], and a study done in iPSC-derived neurons from subjects with SCA3 suggests that excitation of neurons might contribute to the formation of such intermediates [85]. These data demonstrate that L-glutamate-induced excitation of patient-specific iPSC-derived neurons results in ATXN3 cleavage and aggregation that is activitydependent.…”

Plastin 3 is upregulated in iPSC-derived motoneurons from asymptomatic SMN1-deleted individuals

“…In addition, iPCS lines reprogrammed from fibroblasts of an independent SMA I patient [32] and five unrelated control individuals (Online Resource Tab. S1; [33]) were used. Informed written consent was obtained from all patients or care givers according to the Declaration of Helsinki, and the study was approved by the ethics committee of the University Hospital of Cologne.…”

Polyglutamine ataxias: From Clinical and Molecular Features to Current Therapeutic Strategies

“…The same group also showed that intracellular aggregates only occurred when ataxin-3 contained an expanded polyQ repeat, thus providing compelling evidence to the notion that mutant polyQ expansion directly leads to aggregation in the brain [53]. This research taken on its own does not provide definitive backing for the role of the polyQ tract in ataxin-3 aggregation, however, several subsequent studies both in animal and cell models have supported this concept over several different polyQ diseases [54][55][56][57][58][59]. Along with the presence of an expanded polyQ region, the length of the expansion itself is said to play a major role in aggregation.…”

“…However, more recently it appears that fragmented mutant ataxin-3 initially forms the intracellular aggregates and then acts a seed or a catalyst for recruitment of the full-length protein [53,157]. A study showed that L-glutamineinduced excitation of neurons lead to Ca 2+ dependent proteolysis of ataxin-3, creating SDS-insoluble aggregates [57]: suggesting that initial microaggregates catalyse and precede large inclusion bodies that ultimately lead to neural death and rapid disease progression. It was also found that aggregation of the ataxin-3 fragments was calpain dependent and that the phenotypic changes could be abolished by calpain inhibition.…”