Failure to Deliver and Translate—New Insights into RNA Dysregulation in ALS

Amyotrophic Lateral Sclerosis (ALS) is a progressive and fatal neurodegenerative disease affecting both upper and lower motor neurons. The molecular mechanisms underlying disease pathogenesis remain largely unknown. Multiple genetic loci including genes involved in proteostasis and ribostasis have been linked to ALS providing key insights into the molecular mechanisms underlying disease. In particular, the identification of the RNA binding proteins TDP-43 and fused in sarcoma (FUS) as causative factors of ALS resulted in a paradigm shift centered on the study of RNA dysregulation as a major mechanism of disease. With wild-type TDP-43 pathology being found in ~97% of ALS cases and the identification of disease causing mutations within its sequence, TDP-43 has emerged as a prominent player in ALS. More recently, studies of the newly discovered C9orf72 repeat expansion are lending further support to the notion of defects in RNA metabolism as a key factor underlying ALS. RNA binding proteins are involved in all aspects of RNA metabolism ranging from splicing, transcription, transport, storage into RNA/protein granules, and translation. How these processes are affected by disease-associated mutations is just beginning to be understood. Considerable work has gone into the identification of splicing and transcription defects resulting from mutations in RNA binding proteins associated with disease. More recently, defects in RNA transport and translation have been shown to be involved in the pathomechanism of ALS. A central hypothesis in the field is that disease causing mutations lead to the persistence of RNA/protein complexes known as stress granules. Under times of prolonged cellular stress these granules sequester specific mRNAs preventing them from translation, and are thought to evolve into pathological aggregates. Here we will review recent efforts directed at understanding how altered RNA metabolism contributes to ALS pathogenesis

Post-Transcriptional Inhibition Of Hsc70-4/Hspa8 Expression Leads To Synaptic Vesicle Cycling Defects In Multiple Models Of Als

Amyotrophic lateral sclerosis (ALS) is a synaptopathy accompanied by the presence of cytoplasmic aggregates containing TDP-43, an RNA-binding protein linked to ∼97% of ALS cases. Using a Drosophila model of ALS, we show that TDP-43 overexpression (OE) in motor neurons results in decreased expression of the Hsc70-4 chaperone at the neuromuscular junction (NMJ). Mechanistically, mutant TDP-43 sequesters hsc70-4 mRNA and impairs its translation. Expression of the Hsc70-4 ortholog, HSPA8, is also reduced in primary motor neurons and NMJs of mice expressing mutant TDP-43. Electrophysiology, imaging, and genetic interaction experiments reveal TDP-43-dependent defects in synaptic vesicle endocytosis. These deficits can be partially restored by OE of Hsc70-4, cysteine-string protein (Csp), or dynamin. This suggests that TDP-43 toxicity results in part from impaired activity of the synaptic CSP/Hsc70 chaperone complex impacting dynamin function. Finally, Hsc70-4/HSPA8 expression is also post-transcriptionally reduced in fly and human induced pluripotent stem cell (iPSC) C9orf72 models, suggesting a common disease pathomechanism. Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by synaptic failure. Coyne et al. show that in multiple models of ALS, ranging from Drosophila to mice to patient-derived motor neurons, deficits in synaptic vesicle cycling can be explained by dysregulation of the Hsc70-4/HSPA8 chaperone

G4C2 Repeat RNA Initiates a POM121-Mediated Reduction in Specific Nucleoporins in C9orf72 ALS/FTD

Through mechanisms that remain poorly defined, defects in nucleocytoplasmic transport and accumulations of specific nuclear-pore-complex-associated proteins have been reported in multiple neurodegenerative diseases, including C9orf72 Amyotrophic Lateral Sclerosis and Frontotemporal Dementia (ALS/FTD). Using super-resolution structured illumination microscopy, we have explored the mechanism by which nucleoporins are altered in nuclei isolated from C9orf72 induced pluripotent stem-cell-derived neurons (iPSNs). Of the 23 nucleoporins evaluated, we observed a reduction in a subset of 8, including key components of the nuclear pore complex scaffold and the transmembrane nucleoporin POM121. Reduction in POM121 appears to initiate a decrease in the expression of seven additional nucleoporins, ultimately affecting the localization of Ran GTPase and subsequent cellular toxicity in C9orf72 iPSNs. Collectively, our data suggest that the expression of expanded C9orf72 ALS/FTD repeat RNA alone affects nuclear POM121 expression in the initiation of a pathological cascade affecting nucleoporin levels within neuronal nuclei and ultimately downstream neuronal survival

Post-transcriptional Inhibition of Hsc70-4/HSPA8 Expression Leads to Synaptic Vesicle Cycling Defects in Multiple Models of ALS

Amyotrophic lateral sclerosis (ALS) is a synaptopathy accompanied by the presence of cytoplasmic aggregates containing TDP-43, an RNA-binding protein linked to similar to 97% of ALS cases. Using a Drosophila model of ALS, we show that TDP-43 overexpression (OE) in motor neurons results in decreased expression of the Hsc70-4 chaperone at the neuromuscular junction (NMJ). Mechanistically, mutant TDP-43 sequesters hsc70-4 mRNA and impairs its translation. Expression of the Hsc70-4 ortholog, HSPA8, is also reduced in primary motor neurons and NMJs of mice expressing mutant TDP-43. Electrophysiology, imaging, and genetic interaction experiments reveal TDP-43-dependent defects in synaptic vesicle endocytosis. These deficits can be partially restored by OE of Hsc70-4, cysteine-string protein (Csp), or dynamin. This suggests that TDP-43 toxicity results in part from impaired activity of the synaptic CSP/Hsc70 chaperone complex impacting dynamin function. Finally, Hsc70-4/HSPA8 expression is also post-transcriptionally reduced in fly and human induced pluripotent stem cell (iPSC) C9orf72 models, suggesting a common disease pathomechanism.NIH K-INBRE postdoctoral award [P20 GM103418]; NIH [R00 NS082376, R01NS078214, R01AG051470, R21 NS094809, RO1 NS091299]; Emory Medicine Catalyst Award; Muscular Dystrophy Association [MDA348086]; ALS Association ALSA [16-IIP-278]; Barrow Neurological Foundation; Himelic Family Foundation; Neuroscience Graduate Interdisciplinary Program at UA; ARCSThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]