Related Experiment Video
Updated: Jul 14, 2025
Measuring RAN Peptide Toxicity in C. elegans
Published on: April 30, 2020
Divergent Molecular Pathways for Toxicity of Selected Mutant C9ORF72-derived Dipeptide Repeats
Abstract:
Expansion of a hexanucleotide repeat in a noncoding region of the C9ORF72 gene is responsible for a significant fraction of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD) cases, but mechanisms linking mutant gene products to neuronal toxicity remain debatable. Pathogenesis was proposed to involve the production of toxic RNA species and/or accumulation of toxic dipeptide repeats (DPRs) but distinguishing between these mechanisms has been challenging. In this study, we first use complementary model systems for analyzing pathogenesis in adult-onset neurodegenerative diseases to characterize the pathogenicity of DPRs produced by Repeat Associated Non-ATG translation of C9ORF72 in specific cellular compartments: isolated axoplasm and giant synapse from the squid. Results showed selective axonal and presynaptic toxicity of GP-DPRs, independent of associated RNA. These effects involved a MAPK signaling pathway that affects fast axonal transport and synaptic function, a pathogenic mechanism shared with other mutant proteins associated with familial ALS, like SOD1 and FUS. In primary cultured neurons, GP but not other DPRs promote the "dying-back" axonopathy seen in ALS. Interestingly, GR- and PR-DPRs, which had no effect on axonal transport or synaptic transmission, were found to disrupt the nuclear membrane, promoting "dying-forward" neuropathy. All C9-DPR-mediated toxic effects observed in these studies are independent of whether the corresponding mRNAs contained hexanucleotide repeats or alternative codons. Finally, C9ORF72 human tissues confirmed a close association between GP and active P38 in degenerating motor neurons as well as GR-associated nuclear damage in the cortex. Collectively, our studies establish compartment-specific toxic effects of C9-DPRs associated with degeneration, suggesting that two independent pathogenic mechanisms may contribute to disease heterogeneity and/or synergize on disease progression in C9ORF72 patients with ALS and/or FTD symptoms.
Insights
Toxic dipeptide repeats (DPRs) from the C9ORF72 gene cause distinct neuronal damage in Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Dementia (FTD), revealing new insights into neurodegeneration.
Area of Science:
- Neuroscience
- Genetics
- Molecular Biology
Background:
- Hexanucleotide repeat expansion in C9ORF72 is a major cause of ALS and FTD.
- Mechanisms of C9ORF72-linked neurotoxicity, particularly the roles of toxic RNA and dipeptide repeats (DPRs), are not fully understood.
Purpose of the Study:
- To investigate the compartment-specific pathogenicity of DPRs produced by C9ORF72 Repeat Associated Non-ATG translation.
- To elucidate the distinct mechanisms by which different DPRs contribute to neuronal degeneration in ALS and FTD.
Main Methods:
- Utilized complementary model systems, including squid axoplasm and giant synapses, and primary cultured neurons.
- Analyzed the effects of specific DPRs (GP, GR, PR) on axonal transport, synaptic function, and nuclear membrane integrity.
- Examined C9ORF72 human tissues to correlate DPRs with pathological markers in degenerating neurons.
Main Results:
- Glycine-proline DPRs (GP-DPRs) selectively caused axonal and presynaptic toxicity via a MAPK pathway, mimicking "dying-back" axonopathy in ALS.
- Arginine-glycine (GR) and proline-arginine (PR) DPRs disrupted the nuclear membrane, leading to "dying-forward" neuropathy.
- Observed a correlation between GP-DPRs and active P38 in degenerating motor neurons and GR-associated nuclear damage in human tissues.
Conclusions:
- C9ORF72-associated neurodegeneration involves compartment-specific toxic effects of DPRs.
- Distinct pathogenic mechanisms, potentially acting independently or synergistically, contribute to disease heterogeneity in C9ORF72-linked ALS and FTD.
