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Free keywords:
Animals
Animals, Genetically Modified
Cell Nucleolus/*metabolism/*pathology
Cell Nucleus Size/genetics/physiology
DNA Repeat Expansion
Drosophila
Fluorescent Antibody Technique
Frontal Lobe/metabolism/pathology
Frontotemporal Lobar Degeneration/genetics/*metabolism/*pathology
Humans
Imaging, Three-Dimensional
In Situ Hybridization, Fluorescence
Intranuclear Inclusion Bodies/metabolism/pathology
Microscopy, Confocal
Neurons/metabolism/pathology
Proteins/genetics/*metabolism
Stress, Physiological/genetics/physiology
C9orf72
Dipeptide repeat proteins
Ftld
Nucleolar stress
Poly(GR)
RNA foci
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.
