An expanded GGGGCC repeat in C9orf72 is the most common genetic cause of frontotemporal dementia and amyotrophic lateral sclerosis. A fundamental question is whether toxicity is driven by the repeat RNA itself and/or by dipeptide repeat proteins generated by repeat-associated, non-ATG translation. To address this question we developed in vitro and in vivo models to dissect repeat RNA and dipeptide repeat protein toxicity. Expression of pure repeats in Drosophila caused adult-onset neurodegeneration attributable to poly-(glycine-arginine) proteins. Thus expanded repeats promoted neurodegeneration through neurotoxic proteins. Expression of individual dipeptide repeat proteins with a non-GGGGCC RNA sequence showed both poly-(glycinearginine) and poly-(proline-arginine) proteins caused neurodegeneration. These findings are consistent with a dual toxicity mechanism, whereby both arginine-rich proteins and repeat RNA contribute to C9orf72-mediated neurodegeneration.Frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS) are adult-onset, neurodegenerative diseases associated with personality change, language dysfunction and † Corresponding authors. a.isaacs@prion.ucl.ac.uk; l.partridge@ucl.ac.uk. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts progressive muscle weakness. These syndromes overlap genetically and pathologically, and can also co-occur in individuals, and within families (1). An intronic GGGGCC hexanucleotide repeat expansion in C9orf72 is the most common genetic cause of both FTD and ALS (C9FTD/ALS) (2-4), and can be found in patients diagnosed with all common neurodegenerative diseases (5). Healthy individuals carry fewer than 33 hexanucleotide repeats, with 2 repeats being the most common, but C9FTD/ALS cases carry between 400 and 4400 repeats (2, 5, 6).The repeat expansion could cause disease by three possible mechanisms: i) toxic sense and/or antisense repeat RNA species that sequester key RNA-binding proteins, ii) toxic dipeptide repeat (DPR) proteins, generated by repeat-associated, non-ATG (RAN) translation, or iii) reduced expression of C9orf72. The absence of a severe phenotype in a homozygous C9orf72 mutation case (7), and the lack of C9orf72 coding mutations (8) argue against loss-of-function as a primary mechanism. Neuronal aggregates of RNA, termed RNA foci, generated from both sense and antisense repeat transcripts are frequent in C9FTD/ALS patient brain (9-13). The GGGGCC repeat can be translated in all sense and antisense frames, two of which encode the same DPR, resulting in five DPR proteins, all of which form inclusions in widespread brain regions (10,12,(14)(15)(16)(17)(18). It is therefore of fundamental importance to understand the contributions of repeat RNA and DPR proteins to C9orf72-mediated neurodegeneration.A major obstacle in the investigation of large expanded repeats is that they are inherently unstable. We used recombination-deficient E. coli and a cloning strategy termed recursive directional ligati...

Ubiquitin-positive intraneuronal inclusions are a consistent feature of the major human neurodegenerative diseases, suggesting that dysfunction of the ubiquitin proteasome system is central to disease etiology. Research using inhibitors of the 20S proteasome to model Parkinson's disease is controversial. We report for the first time that specifically 26S proteasomal dysfunction is sufficient to trigger neurodegenerative disease. Here, we describe novel conditional genetic mouse models using the Cre/loxP system to spatially restrict inactivation of Psmc1 (Rpt2/S4) to neurons of either the substantia nigra or forebrain (e.g., cortex, hippocampus, and striatum). PSMC1 is an essential subunit of the 26S proteasome and Psmc1 conditional knock-out mice display 26S proteasome depletion in targeted neurons, in which the 20S proteasome is not affected. Impairment of specifically ubiquitin-mediated protein degradation caused intraneuronal Lewy-like inclusions and extensive neurodegeneration in the nigrostriatal pathway and forebrain regions. Ubiquitin and ␣-synuclein neuropathology was evident, similar to human Lewy bodies, but interestingly, inclusion bodies contained mitochondria. We support this observation by demonstrating mitochondria in an early form of Lewy body (pale body) from Parkinson's disease patients. The results directly confirm that 26S dysfunction in neurons is involved in the pathology of neurodegenerative disease. The model demonstrates that 26S proteasomes are necessary for normal neuronal homeostasis and that 20S proteasome activity is insufficient for neuronal survival. Finally, we are providing the first reproducible genetic platform for identifying new therapeutic targets to slow or prevent neurodegeneration.

Devoy et al. develop the first mouse model to fully recapitulate human FUS-ALS, as defined by midlife-onset progressive degeneration of motor neurons with dominant inheritance. A toxic gain of function occurs in the absence of FUS protein aggregation, involving disturbance of ribosomes and mitochondria at the endoplasmic reticulum.

The charged multivesicular body proteins (Chmp1-7) are an evolutionarily conserved family of cytosolic proteins that transiently assembles into helical polymers that change the curvature of cellular membrane domains. Mutations in human CHMP2B cause frontotemporal dementia, suggesting that this protein may normally control some neuron-specific process. Here, we examined the function, localization, and interactions of neuronal Chmp2b. The protein was highly expressed in mouse brain and could be readily detected in neuronal dendrites and spines. Depletion of endogenous Chmp2b reduced dendritic branching of cultured hippocampal neurons, decreased excitatory synapse density in vitro and in vivo, and abolished activity-induced spine enlargement and synaptic potentiation. To understand the synaptic effects of Chmp2b, we determined its ultrastructural distribution by quantitative immuno-electron microscopy and its biochemical interactions by coimmunoprecipitation and mass spectrometry. In the hippocampus in situ, a subset of neuronal Chmp2b was shown to concentrate beneath the perisynaptic membrane of dendritic spines. In synaptoneurosome lysates, Chmp2b was stably bound to a large complex containing other members of the Chmp family, as well as postsynaptic scaffolds. The supramolecular Chmp assembly detected here corresponds to a stable form of the endosomal sorting complex required for transport-III (ESCRT-III), a ubiquitous cytoplasmic protein complex known to play a central role in remodeling of lipid membranes. We conclude that Chmp2b-containing ESCRT-III complexes are also present at dendritic spines, where they regulate synaptic plasticity. We propose that synaptic ESCRT-III filaments may function as a novel element of the submembrane cytoskeleton of spines.

Abstract Mutations in the RNA-binding protein FUS cause amyotrophic lateral sclerosis (ALS), a devastating neurodegenerative disease. FUS plays a role in numerous aspects of RNA metabolism, including mRNA splicing. However, the impact of ALS-causative mutations on splicing has not been fully characterized, as most disease models have been based on overexpressing mutant FUS, which will alter RNA processing due to FUS autoregulation. We and others have recently created knockin models that overcome the overexpression problem, and have generated high depth RNA-sequencing on FUS mutants in parallel to FUS knockout, allowing us to compare mutation-induced changes to genuine loss of function. We find that FUS-ALS mutations induce a widespread loss of function on expression and splicing. Specifically, we find that mutant FUS directly alters intron retention levels in RNA-binding proteins. Moreover, we identify an intron retention event in FUS itself that is associated with its autoregulation. Altered FUS levels have been linked to disease, and we show here that this novel autoregulation mechanism is altered by FUS mutations. Crucially, we also observe this phenomenon in other genetic forms of ALS, including those caused by TDP-43, VCP and SOD1 mutations, supporting the concept that multiple ALS genes interact in a regulatory network.