The causes of amyotrophic lateral sclerosis (ALS), a devastating human neurodegenerative disease, are poorly understood, although the protein TDP-43 has been suggested to have a critical role in disease pathogenesis. Here we show that ataxin 2 (ATXN2), a polyglutamine (polyQ) protein mutated in spinocerebellar ataxia type 2, is a potent modifier of TDP-43 toxicity in animal and cellular models. ATXN2 and TDP-43 associate in a complex that depends on RNA. In spinal cord neurons of ALS patients, ATXN2 is abnormally localized; likewise, TDP-43 shows mislocalization in spinocerebellar ataxia type 2. To assess the involvement of ATXN2 in ALS, we analysed the length of the polyQ repeat in the ATXN2 gene in 915 ALS patients. We found that intermediate-length polyQ expansions (27-33 glutamines) in ATXN2 were significantly associated with ALS. These data establish ATXN2 as a relatively common ALS susceptibility gene. Furthermore, these findings indicate that the TDP-43-ATXN2 interaction may be a promising target for therapeutic intervention in ALS and other TDP-43 proteinopathies.

Amyotrophic lateral sclerosis (ALS) is a rapidly progressing neurodegenerative disease that is characterized by motor neuron loss and that leads to paralysis and death 2-5 years after disease onset. Nearly all patients with ALS have aggregates of the RNA-binding protein TDP-43 in their brains and spinal cords, and rare mutations in the gene encoding TDP-43 can cause ALS. There are no effective TDP-43-directed therapies for ALS or related TDP-43 proteinopathies, such as frontotemporal dementia. Antisense oligonucleotides (ASOs) and RNA-interference approaches are emerging as attractive therapeutic strategies in neurological diseases. Indeed, treatment of a rat model of inherited ALS (caused by a mutation in Sod1) with ASOs against Sod1 has been shown to substantially slow disease progression. However, as SOD1 mutations account for only around 2-5% of ALS cases, additional therapeutic strategies are needed. Silencing TDP-43 itself is probably not appropriate, given its critical cellular functions. Here we present a promising alternative therapeutic strategy for ALS that involves targeting ataxin-2. A decrease in ataxin-2 suppresses TDP-43 toxicity in yeast and flies, and intermediate-length polyglutamine expansions in the ataxin-2 gene increase risk of ALS. We used two independent approaches to test whether decreasing ataxin-2 levels could mitigate disease in a mouse model of TDP-43 proteinopathy. First, we crossed ataxin-2 knockout mice with TDP-43 (also known as TARDBP) transgenic mice. The decrease in ataxin-2 reduced aggregation of TDP-43, markedly increased survival and improved motor function. Second, in a more therapeutically applicable approach, we administered ASOs targeting ataxin-2 to the central nervous system of TDP-43 transgenic mice. This single treatment markedly extended survival. Because TDP-43 aggregation is a component of nearly all cases of ALS, targeting ataxin-2 could represent a broadly effective therapeutic strategy.

Astrocytes regulate the response of the central nervous system to disease and injury and have been hypothesized to actively kill neurons in neurodegenerative disease1-6. Here we report an approach to isolate one component of the long-sought astrocyte-derived toxic factor5,6. Notably, instead of a protein, saturated lipids contained in APOE and APOJ lipoparticles mediate astrocyte-induced toxicity. Eliminating the formation of long-chain saturated lipids by astrocyte-specific knockout of the saturated lipid synthesis enzyme ELOVL1 mitigates astrocyte-mediated toxicity in vitro as well as in a model of acute axonal injury in vivo. These results suggest a mechanism by which astrocytes kill cells in the central nervous system.

Algorithms designed to identify canonical yeast prions predict that around 250 human proteins, including several RNA-binding proteins associated with neurodegenerative disease, harbour a distinctive prion-like domain (PrLD) enriched in uncharged polar amino acids and glycine. PrLDs in RNA-binding proteins are essential for the assembly of ribonucleoprotein granules. However, the interplay between human PrLD function and disease is not understood. Here we define pathogenic mutations in PrLDs of heterogeneous nuclear ribonucleoproteins (hnRNPs) A2B1 and A1 in families with inherited degeneration affecting muscle, brain, motor neuron and bone, and in one case of familial amyotrophic lateral sclerosis. Wild-type hnRNPA2 (the most abundant isoform of hnRNPA2B1) and hnRNPA1 show an intrinsic tendency to assemble into self-seeding fibrils, which is exacerbated by the disease mutations. Indeed, the pathogenic mutations strengthen a 'steric zipper' motif in the PrLD, which accelerates the formation of self-seeding fibrils that cross-seed polymerization of wild-type hnRNP. Notably, the disease mutations promote excess incorporation of hnRNPA2 and hnRNPA1 into stress granules and drive the formation of cytoplasmic inclusions in animal models that recapitulate the human pathology. Thus, dysregulated polymerization caused by a potent mutant steric zipper motif in a PrLD can initiate degenerative disease. Related proteins with PrLDs should therefore be considered candidates for initiating and perhaps propagating proteinopathies of muscle, brain, motor neuron and bone.

A hallmark pathological feature of the neurodegenerative diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is the depletion of RNA-binding protein TDP-43 from the nucleus of neurons in the brain and spinal cord1. A major function of TDP-43 is as a repressor of cryptic exon inclusion during RNA splicing2-4. Single nucleotide polymorphisms in UNC13A are among the strongest hits associated with FTD and ALS in human genome-wide association studies5,6, but how those variants increase risk for disease is unknown. Here we show that TDP-43 represses a cryptic exon-splicing event in UNC13A. Loss of TDP-43 from the nucleus in human brain, neuronal cell lines and motor neurons derived from induced pluripotent stem cells resulted in the inclusion of a cryptic exon in UNC13A mRNA and reduced UNC13A protein expression. The top variants associated with FTD or ALS risk in humans are located in the intron harbouring the cryptic exon, and we show that they increase UNC13A cryptic exon splicing in the face of TDP-43 dysfunction. Together, our data provide a direct functional link between one of the strongest genetic risk factors for FTD and ALS (UNC13A genetic variants), and loss of TDP-43 function.