Aggregation-prone c9FTD/ALS poly(GA) RAN-translated proteins cause neurotoxicity by inducing ER stress

Abstract

The occurrence of repeat-associated non-ATG (RAN) translation, an atypical form of translation of expanded repeats that results in the synthesis of homopolymeric expansion proteins, is becoming more widely appreciated among microsatellite expansion disorders. Such disorders include amyotrophic lateral sclerosis and frontotemporal dementia caused by a hexanucleotide repeat expansion in the C9ORF72 gene (c9FTD/ALS). We and others have recently shown that this bidirectionally transcribed repeat is RAN translated, and the “c9RAN proteins” thusly produced form neuronal inclusions throughout the central nervous system of c9FTD/ALS patients. Nonetheless, the potential contribution of c9RAN proteins to disease pathogenesis remains poorly understood. In the present study, we demonstrate that poly(GA) c9RAN proteins are neurotoxic and may be implicated in the neurodegenerative processes of c9FTD/ALS. Specifically, we show that expression of poly(GA) proteins in cultured cells and primary neurons leads to the formation of soluble and insoluble high molecular weight species, as well as inclusions composed of filaments similar to those observed in c9FTD/ALS brain tissues. The expression of poly(GA) proteins is accompanied by caspase-3 activation, impaired neurite outgrowth, inhibition of proteasome activity, and evidence of endoplasmic reticulum (ER) stress. Of importance, ER stress inhibitors, salubrinal and TUDCA, provide protection against poly(GA)-induced toxicity. Taken together, our data provide compelling evidence towards establishing RAN translation as a pathogenic mechanism of c9FTD/ALS, and suggest that targeting the ER using small molecules may be a promising therapeutic approach for these devastating diseases.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-014-1336-5) contains supplementary material, which is available to authorized users.

Timing and significance of pathological features inC9orf72expansion-associated frontotemporal dementia

“…Expanded repeats also undergo abnormal repeat associated non-ATG (RAN) translation (Zu et al, 2011). In patients, these proteins form stellate cytoplasmic inclusions in degenerate and non-degenerate brain regions Mackenzie et al, 2013;Mann et al, 2013;Mori et al, 2013b;Zu et al, 2013) and some have been shown to disrupt cellular function and cause toxicity when overexpressed in model systems (Kwon et al, 2014;Mizielinska et al, 2014;Wen et al, 2014;Zhang et al, 2014). Due to the number and complexity of these potential mechanisms, it remains unclear which among them contributes most to disease pathogenesis in patients.…”

ER stress in neurodegenerative disease: from disease mechanisms to therapeutic interventions

“…Similarly, gene expression analysis of ALS brain tissue from patients carrying C9orf72 mutations, the most relevant genetic alteration in ALS, mutations shows major alteration in ER stress-related genes in cerebellum [116]. In agreement with this, expression of RAN peptides derived from C9orf72 mutation in cell culture also triggers abnormal levels of ER stress [117].…”

50 & 100 Years Ago

“…Indeed, biochemical data suggested that as many as 50% of the proteasome complexes in the neuron become highly entangled within poly(GA) ribbons. Removal of proteasomes from their normal location in cells through this sequestration mechanism might explain the reduced proteasomal activity in cells harbouring these aggregates 4,8 . Complexes called ribosomes, which mediate protein production and are comparable in size to proteasomes, were largely excluded from poly(GA) ribbons, suggesting that poly(GA) aggregates are actively recruited or retained by proteasomes.…”