Abstract
An intronic GGGGCC expansion in C9orf72 is the most common known cause of both frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). The repeat expansion leads to the generation of sense and antisense repeat RNA aggregates and dipeptide repeat (DPR) proteins, generated by repeat-associated non-ATG translation. The arginine-rich DPR proteins poly(glycine-arginine or GR) and poly(proline-arginine or PR) are potently neurotoxic and can localise to the nucleolus when expressed in cells, resulting in enlarged nucleoli with disrupted functionality. Furthermore, GGGGCC repeat RNA can bind nucleolar proteins in vitro. However, the relevance of nucleolar stress is unclear, as the arginine-rich DPR proteins do not localise to the nucleolus in C9orf72-associated FTLD/ALS (C9FTLD/ALS) patient brain. We measured nucleolar size in C9FTLD frontal cortex neurons using a three-dimensional, volumetric approach. Intriguingly, we found that C9FTLD brain exhibited bidirectional nucleolar stress. C9FTLD neuronal nucleoli were significantly smaller than control neuronal nucleoli. However, within C9FTLD brains, neurons containing poly(GR) inclusions had significantly larger nucleolar volumes than neurons without poly(GR) inclusions. In addition, expression of poly(GR) in adult Drosophila neurons led to significantly enlarged nucleoli. A small but significant increase in nucleolar volume was also observed in C9FTLD frontal cortex neurons containing GGGGCC repeat-containing RNA foci. These data show that nucleolar abnormalities are a consistent feature of C9FTLD brain, but that diverse pathomechanisms are at play, involving both DPR protein and repeat RNA toxicity.
Highlights
An intronic GGGGCC expansion in C9orf72 is the most common known cause of both frontotemporal lobar dementia (FTLD) and amyotrophic lateral sclerosis (ALS) [7, 27]
Bidirectional changes in nucleolar volume dependent on the presence or absence of C9orf72 repeat RNA or protein pathologies show the heterogeneity of pathomechanisms in patient neurons, but support findings in current experimental models and have important implications for understanding the complex disease processes involved in C9FTLD/ALS
Frequency distribution of pooled control and C9FTLD nucleolin volumes were similar (c) and median nucleolin volume was no different in neurons from C9FTLD cases and controls (d). e Quantification of neuronal nuclear volume determined by d Correlation of nucleophosmin and nuclear (DAPI) staining
Summary
An intronic GGGGCC expansion in C9orf is the most common known cause of both frontotemporal lobar dementia (FTLD) and amyotrophic lateral sclerosis (ALS) [7, 27]. Overexpression of poly(GR) or poly(PR) repeats in cell models leads to their localisation in the nucleolus, and results in enlarged nucleoli and altered ribosomal RNA processing [13, 32, 34]. Dispersal of the nucleolar protein nucleolin was observed within neurons of adult C9orf BAC transgenic mice, but no consequent change in ribosomal RNA biogenesis was detected [23]. Enlarged nucleoli and altered ribosomal RNA processing have been reported in cells derived from patients with a C9orf repeat expansion, including lymphocytes, fibroblasts and induced pluripotent stem cells differentiated into neurons [10]. Disruption of nucleolar structure and ribosomal RNA transcription have been reported in several neurodegenerative diseases, both in post-mortem patient tissue and animal models [25]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
