TDP-43 suppresses CGG repeat-induced neurotoxicity through interactions with HnRNP A2/B1

Abstract

Nucleotide repeat expansions can elicit neurodegeneration as RNA by sequestering specific RNA-binding proteins, preventing them from performing their normal functions. Conversely, mutations in RNA-binding proteins can trigger neurodegeneration at least partly by altering RNA metabolism. In Fragile X-associated tremor/ataxia syndrome (FXTAS), a CGG repeat expansion in the 5'UTR of the fragile X gene (FMR1) leads to progressive neurodegeneration in patients and CGG repeats in isolation elicit toxicity in Drosophila and other animal models. Here, we identify the amyotrophic lateral sclerosis (ALS)-associated RNA-binding protein TAR DNA-binding protein (TDP-43) as a suppressor of CGG repeat-induced toxicity in a Drosophila model of FXTAS. The rescue appears specific to TDP-43, as co-expression of another ALS-associated RNA-binding protein, FUS, exacerbates the toxic effects of CGG repeats. Suppression of CGG RNA toxicity was abrogated by disease-associated mutations in TDP-43. TDP-43 does not co-localize with CGG RNA foci and its ability to bind RNA is not required for rescue. TDP-43-dependent rescue does, however, require fly hnRNP A2/B1 homologues Hrb87F and Hrb98DE. Deletions in the C-terminal domain of TDP-43 that preclude interactions with hnRNP A2/B1 abolish TDP-43-dependent rescue of CGG repeat toxicity. In contrast, suppression of CGG repeat toxicity by hnRNP A2/B1 is not affected by RNAi-mediated knockdown of the fly TDP-43 orthologue, TBPH. Lastly, TDP-43 suppresses CGG repeat-triggered mis-splicing of an hnRNP A2/B1-targeted transcript. These data support a model in which TDP-43 suppresses CGG-mediated toxicity through interactions with hnRNP A2/B1 and suggest a convergence of pathogenic cascades between repeat expansion disorders and RNA-binding proteins implicated in neurodegenerative disease.

Regulatory Role of RNA Chaperone TDP-43 for RNA Misfolding and Repeat-Associated Translation in SCA31

“…Previous reports suggested that TDP-43 suppression of CGG repeat toxicity is dependent on hnRNPA2B1 orthologues in an FXTAS fly model (He et al, 2014). On the other hand, hnRNPA2B1 suppression of CGG repeat toxicity was reported to be independent of a TDP-43 orthologue (He et al, 2014).…”

A fruitful endeavor: Modeling ALS in the fruit fly

“…Interestingly, it was found that ALS-causing mutations of TDP-43 enhanced the phenotypes associated with FXTAS. In addition, phenotypic suppression by TDP-43 appears to be through an interaction with Heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1), suggesting a link between these two RNA-binding proteins and FXTAS (He et al, 2014). …”

Genetic and Pathological Assessment of hnRNPA1, hnRNPA2/B1, and hnRNPA3 in Familial and Sporadic Amyotrophic Lateral Sclerosis

“…The same group also reported that a reduction in nuclear hnRNPA3 leads to an increased level of C9orf72 repeat RNA foci and dipeptide protein repeats in ALS/FTD and ALS patient hippocampal tissues, as well as in ALS patient fibroblasts positive for the expansion [9]. In neurons and oligodendrocytes, hnRNPA3 has an affinity to the 21-nt hnRNPA2 response element (A2RE), which is essential for the export of several mRNAs from the nucleus to the cytoplasm [22,23]. In the neurites of cultured hippocampal neurons, microinjected A2RE-RNA was found to co-localise with hnRNPA3 in cytoplasmic RNA granules, suggesting a role of hnRNPA3 in RNA trafficking [22].…”