CGG repeats in RNA modulate expression of TDP-43 in mouse and fly models of fragile X tremor ataxia syndrome

Abstract

Determining the molecular mechanism(s) leading to Purkinje neuron loss in the neurodegenerative disorder fragile X-associated tremor/ataxia syndrome (FXTAS) is limited by the complex morphology of this cell type. Purkinje neurons are notoriously difficult to isolate and maintain in culture presenting considerable difficultly to identify molecular changes in response to expanded CGG repeat (rCGG)-containing mRNA that induces neurotoxicity in FXTAS. Several studies have uncovered a number of RNA-binding proteins involved in translation that aberrantly interact with the CGG-containing RNA; however, whether these interactions alter the translational profile of cells has not been investigated. Here we employ bacTRAP translational profiling to demonstrate that Purkinje neurons ectopically expressing 90 CGG repeats exhibit a dramatic change in their translational profile even prior to the onset of rCGG-induced phenotypes. This approach identified ∼500 transcripts that are differentially associated with ribosomes in r(CGG)₉₀-expressing mice. Functional annotation cluster analysis revealed broad ontologies enriched in the r(CGG)₉₀ list, including RNA binding and response to stress. Intriguingly, a transcript for the Tardbp gene, implicated in a number of other neurodegenerative disorders, exhibits altered association with ribosomes in the presence of r(CGG)₉₀ repeats. We therefore tested and showed that reduced association of Tardbp mRNA with the ribosomes results in a loss of TDP-43 protein expression in r(CGG)₉₀-expressing Purkinje neurons. Furthermore, we showed that TDP-43 could modulate the rCGG repeat-mediated toxicity in a Drosophila model that we developed previously. These findings together suggest that translational dysregulation may be an underlying mechanism of rCGG-induced neurotoxicity in FXTAS.

Repeat-associated non-AUG (RAN) translation and other molecular mechanisms in Fragile X Tremor Ataxia Syndrome

“…The RNA and DNA binding protein TDP-43 is one such protein that is of special interest for neurodegenerative diseases as protein inclusions of TDP-43 in neurons are a histopathological hallmark of amyotrophic lateral sclerosis and some forms of frontotemporal dementia ( Neumann et al, 2006 ). Two independent studies found that overexpression of TDP-43 partially rescues the neurodegeneration induced by expanded CGG repeats in a Drosophila model of FXTAS, despite the fact that expression of this protein in isolation is toxic itself ( Galloway et al, 2014 , He et al, 2014 ). TDP-43 is not found within CGG repeat RNA aggregates and does not bind the RNA directly; rather, TDP-43 titrates the hnRNP A2/B1 homologue proteins Hrb87F and Hrb98DE away from the repeats, altering their distribution and helping to restore their function ( He et al, 2014 ).…”

A molecular network of the aging brain implicatesINPPL1andPLXNB1in Alzheimer’s disease

“…We downloaded 18 large appropriate datasets from GEO on the Affymetrix 430 GeneChip (microarray) platform, which were applied to measure gene expression in a range of common cell types in the mouse brain [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] . These cell types were captured by a range of methods, including fluorescence assisted cell sorting and laser capture microdissection.…”

RNA-Processing Protein TDP-43 Regulates FOXO-Dependent Protein Quality Control in Stress Response

“…TAR-DNA binding protein (TDP-43) is an RNA-binding protein that has been suggested to play a major role in the pathogenesis of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) [6] – [34] . Bearing features of a heterogeneous nuclear ribonucleoprotein (hnRNP), TDP-43 has well-characterized RNA-processing functions [35] – [37] .…”