C9orf72 Dipeptide Proteins Play a Key Role in Pathogenesis of Amyotrophic Lateral Sclerosis by Disruption of Neuron Function

Mario Tovar

Introduction. Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by progressive corticospinal motor neuron degeneration resulting in death due to respiratory failure within 2-5 years from diagnosis (1,2).  ALS has a mean onset age of 55 years old with documented cases as early as the first and second decades of life (2).  Currently ALS is the most prevalent adult-onset motor neuron disease with an incidence of 1.7 per 100,000 (1).  Studies have revealed an increasingly complex pathogenesis behind ALS and the role C9orf72 gain-of-function products play in ALS motor neuron degeneration (1-3).  Studies have found that hexanucleotide repeat expansion within C9orf72 leads to production of polyGR and polyPR DPRs leading to translational arrest in Drosophila and human iPSC-derived motor neuron models (4,5).  Translation initiation factor eIF1A has been shown in vivo to rescue DPR-induced toxicity (4)  Methods. Drosophila and iPSC-derived motor neuron model was utilized. DPR production was induced in an Drosophila model and then utilizing tandem affinity purification in addition to mass spectrometry to characterize neuronal DPR interactome (4).  Identification of arginine rich DPRs binding ribosomal proteins was identified (4).  Drosophila and were then used as an in vivo model by induced production of DPRs to observe neuronal toxicity (4).  iPSC-derived motor neuron model were then subsequently transfected with plasmids encoding N-terminally tagged arginine-rich DPR proteins; that were initially starved of methionine and later fed L-azidohomoalanine (AHA) to further observe DPR mediated neuronal toxicity (4).  Results. In both Drosophila and iPSC-derived motor neuron models arginine rich DPRs demonstrated neuron toxicity mediated by binding ribosomal proteins and inducing translational arrest (4).  Tandem affinity purification coupled with mass spectrometry was utilized in Drosophila models to reveal that polyPR and polyGR bind ribosomal proteins and proteins necessary for translation (4-6).  The neuronal effects were replicated in vivo models of both Drosophila and iPSC-derived motor neuron demonstrating severely impaired translation activity though arginine rich DPR binding. Both eIF1A and artificial microRNAs have demonstrated therapeutic benefit through reduction of C9orf72 gain-of-function mechanism (5,6).  Conclusion. Increased levels of arginine rich DPRs induced neuronal translational arrest leading to disruption of cellular homeostasis. Highlighting the potential therapeutic avenue of inhibiting C9orf72 gain-of-function mechanisms. Utilization of eIF1A and artificial microRNAs could provide protection to neurons reducing the degree of insult received and ameliorate ALS symptoms.


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