Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by the selective loss of upper and lower motor neurons, a cell type that is intrinsically more vulnerable than other cell types to exogenous stress. The interplay between genetic susceptibility and environmental exposures to toxins has long been thought to be relevant to ALS. One cellular mechanism to overcome stress is the formation of small dense cytoplasmic domains called stress granules (SG) which contain translationally arrested mRNAs. TDP-43 (encoded by TARDBP) is an ALS-causative gene that we have previously implicated in the regulation of the core stress granule proteins G3BP and TIA-1. TIA-1 and G3BP localize to SG under nearly all stress conditions and are considered essential to SG formation. Here, we report that TDP-43 is required for proper SG dynamics, especially SG assembly as marked by the secondary aggregation of TIA-1. We also show that SG assembly, but not initiation, requires G3BP. Furthermore, G3BP can rescue defective SG assembly in cells depleted of endogenous TDP-43. We also demonstrate that endogenous TDP-43 and FUS do not have overlapping functions in this cellular process as SG initiation and assembly occur normally in the absence of FUS. Lastly, we observe that SG assembly is a contributing factor in the survival of neuronal-like cells responding to acute oxidative stress. These data raise the possibility that disruptions of normal stress granule dynamics by loss of nuclear TDP-43 function may contribute to neuronal vulnerability in ALS.

See Fratta and Isaacs (doi:10.1093/brain/awy091) for a scientific commentary on this article.The RNA binding proteins TDP-43 (encoded by TARDBP) and hnRNP A1 (HNRNPA1) are each mutated in certain amyotrophic lateral sclerosis cases and are often mislocalized in cytoplasmic aggregates within motor neurons of affected patients. Cytoplasmic inclusions of TDP-43, which are accompanied by a depletion of nuclear TDP-43, are observed in most amyotrophic lateral sclerosis cases and nearly half of frontotemporal dementia cases. Here, we report that TDP-43 binds HNRNPA1 pre-mRNA and modulates its splicing, and that depletion of nuclear TDP-43 results in increased inclusion of a cassette exon in the HNRNPA1 transcript, and consequently elevated protein levels of an isoform containing an elongated prion-like domain, referred to as hnRNP A1B. Combined in vivo and in vitro approaches demonstrated greater fibrillization propensity for hnRNP A1B, which drives protein aggregation and is toxic to cells. Moreover, amyotrophic lateral sclerosis patients with documented TDP-43 pathology showed neuronal hnRNP A1B cytoplasmic accumulation, indicating that TDP-43 mislocalization may contribute to neuronal vulnerability and loss via altered HNRNPA1 pre-mRNA splicing and function. Given that TDP-43 and hnRNP A1 each bind, and thus modulate, a third of the transcriptome, our data suggest a much broader disruption in RNA metabolism than previously considered.

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