Martin H. Schludi

Bidirectional transcripts of the expanded C9orf72 hexanucleotide repeat are translated into aggregating dipeptide repeat proteins.

Massive GGGGCC repeat expansion in the first intron of the gene C9orf72 is the most common known cause of familial frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Despite its intronic localization and lack of an ATG start codon, the repeat region is translated in all three reading frames into aggregating dipeptide-repeat (DPR) proteins, poly-(Gly-Ala), poly-(Gly-Pro) and poly-(Gly-Arg). We took an antibody-based approach to further validate the translation of DPR proteins. To test whether the antisense repeat RNA transcript is also translated, we raised antibodies against the predicted products, poly-(Ala-Pro) and poly-(Pro-Arg). Both antibodies stained p62-positive neuronal cytoplasmic inclusions throughout the cerebellum and hippocampus indicating that not only sense but also antisense strand repeats are translated into DPR proteins in the absence of ATG start codons. Protein products of both strands co-aggregate suggesting concurrent translation of both strands. Moreover, an antibody targeting the putative carboxyl terminus of DPR proteins can detect inclusion pathology in C9orf72 repeat expansion carriers suggesting that the non-ATG translation continues through the entire repeat and beyond. A highly sensitive monoclonal antibody against poly-(Gly-Arg), visualized abundant inclusion pathology in all cortical regions and some inclusions also in motoneurons. Together, our data show that the GGGGCC repeat is bidirectionally translated into five distinct DPR proteins that co-aggregate in the characteristic p62-positive TDP-43 negative inclusions found in FTLD/ALS cases with C9orf72 repeat expansion. Novel monoclonal antibodies against poly-(Gly-Arg) will facilitate pathological diagnosis of C9orf72 FTLD/ALS.

Distribution of dipeptide repeat proteins in cellular models and C9orf72 mutation cases suggests link to transcriptional silencing

A massive expansion of a GGGGCC repeat upstream of the C9orf72 coding region is the most common known cause of amyotrophic lateral sclerosis and frontotemporal dementia. Despite its intronic localization and lack of a canonical start codon, both strands are translated into aggregating dipeptide repeat (DPR) proteins: poly-GA, poly-GP, poly-GR, poly-PR and poly-PA. To address conflicting findings on the predominant toxicity of the different DPR species in model systems, we compared the expression pattern of the DPR proteins in rat primary neurons and postmortem brain and spinal cord of C9orf72 mutation patients. Only poly-GA overexpression closely mimicked the p62-positive neuronal cytoplasmic inclusions commonly observed for all DPR proteins in patients. In contrast, overexpressed poly-GR and poly-PR formed nucleolar p62-negative inclusions. In patients, most of the less common neuronal intranuclear DPR inclusions were para-nucleolar and p62 positive. Neuronal nucleoli in C9orf72 cases showed normal size and morphology regardless of the presence of poly-GR and poly-PR inclusions arguing against widespread nucleolar stress, reported in cellular models. Colocalization of para-nucleolar DPR inclusions with heterochromatin and a marker of transcriptional repression (H3K9me2) indicates a link to gene transcription. In contrast, we detected numerous intranuclear DPR inclusions not associated with nucleolar structures in ependymal and subependymal cells. In patients, neuronal inclusions of poly-GR, poly-GP and the poly-GA interacting protein Unc119 were less abundant than poly-GA inclusions, but showed similar regional and subcellular distribution. Regardless of neurodegeneration, all inclusions were most abundant in neocortex, hippocampus and thalamus, with few inclusions in brain stem and spinal cord. In the granular cell layer of the cerebellum, poly-GA and Unc119 inclusions were significantly more abundant in cases with FTLD than in cases with MND and FTLD/MND. Poly-PR inclusions were rare throughout the brain but significantly more abundant in the CA3/4 region of FTLD cases than in MND cases. Thus, although DPR distribution is not correlated with neurodegeneration spatially, it correlates with neuropathological subtypes.

Cytoplasmic poly-GA aggregates impair nuclear import of TDP-43 in C9orf72 ALS/FTLD.

Abstract A repeat expansion in the non-coding region of C9orf72 gene is the most common mutation causing frontotemporal lobar degeneration (FTLD) and amyotrophic lateral sclerosis (ALS). Sense and antisense transcripts are translated into aggregating dipeptide repeat (DPR) proteins in all reading frames (poly-GA,-GP,-GR,-PA and -PR) through an unconventional mechanism. How these changes contribute to cytoplasmic mislocalization and aggregation of TDP-43 and thereby ultimately leading to neuron loss remains unclear. The repeat RNA itself and poly-GR/PR have been linked to impaired nucleocytoplasmic transport. Here, we show that compact cytoplasmic poly-GA aggregates impair nuclear import of a reporter containing the TDP-43 nuclear localization (NLS) signal. However, a reporter containing a non-classical PY-NLS was not affected. Moreover, poly-GA expression prevents TNFα induced nuclear translocation of p65 suggesting that poly-GA predominantly impairs the importin-α/β-dependent pathway. In neurons, prolonged poly-GA expression induces partial mislocalization of TDP-43 into cytoplasmic granules. Rerouting poly-GA to the nucleus prevented TDP-43 mislocalization, suggesting a cytoplasmic mechanism. In rescue experiments, expression of importin-α (KPNA3, KPNA4) or nucleoporins (NUP54, NUP62) restores the nuclear localization of the TDP reporter. Taken together, inhibition of nuclear import of TDP-43 by cytoplasmic poly-GA inclusions causally links the two main aggregating proteins in C9orf72 ALS/FTLD pathogenesis.

Poly-GP in cerebrospinal fluid links C9orf72-associated dipeptide repeat expression to the asymptomatic phase of ALS/FTD.

The C9orf72 GGGGCC repeat expansion is a major cause of amyotrophic lateral sclerosis and frontotemporal dementia (c9ALS/FTD). Non‐conventional repeat translation results in five dipeptide repeat proteins (DPRs), but their clinical utility, overall significance, and temporal course in the pathogenesis of c9ALS/FTD are unclear, although animal models support a gain‐of‐function mechanism. Here, we established a poly‐GP immunoassay from cerebrospinal fluid (CSF) to identify and characterize C9orf72 patients. Significant poly‐GP levels were already detectable in asymptomatic C9orf72 mutation carriers compared to healthy controls and patients with other neurodegenerative diseases. The poly‐GP levels in asymptomatic carriers were similar to symptomatic c9ALS/FTD cases. Poly‐GP levels were not correlated with disease onset, clinical scores, and CSF levels of neurofilaments as a marker for axonal damage. Poly‐GP determination in CSF revealed a C9orf72 mutation carrier in our cohort and may thus be used as a diagnostic marker in addition to genetic testing to screen patients. Presymptomatic expression of poly‐GP and likely other DPR species may contribute to disease onset and thus represents an alluring therapeutic target.

Spinal poly-GA inclusions in a C9orf72 mouse model trigger motor deficits and inflammation without neuron loss

Translation of the expanded (ggggcc)n repeat in C9orf72 patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) causes abundant poly-GA inclusions. To elucidate their role in pathogenesis, we generated transgenic mice expressing codon-modified (GA)149 conjugated with cyan fluorescent protein (CFP). Transgenic mice progressively developed poly-GA inclusions predominantly in motoneurons and interneurons of the spinal cord and brain stem and in deep cerebellar nuclei. Poly-GA co-aggregated with p62, Rad23b and the newly identified Mlf2, in both mouse and patient samples. Consistent with the expression pattern, 4-month-old transgenic mice showed abnormal gait and progressive balance impairment, but showed normal hippocampus-dependent learning and memory. Apart from microglia activation we detected phosphorylated TDP-43 but no neuronal loss. Thus, poly-GA triggers behavioral deficits through inflammation and protein sequestration that likely contribute to the prodromal symptoms and disease progression of C9orf72 patients.

TDP-43 loss of function inhibits endosomal trafficking and alters trophic signaling in neurons

Nuclear clearance of TDP‐43 into cytoplasmic aggregates is a key driver of neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), but the mechanisms are unclear. Here, we show that TDP‐43 knockdown specifically reduces the number and motility of RAB11‐positive recycling endosomes in dendrites, while TDP‐43 overexpression has the opposite effect. This is associated with delayed transferrin recycling in TDP‐43‐knockdown neurons and decreased β2‐transferrin levels in patient CSF. Whole proteome quantification identified the upregulation of the ESCRT component VPS4B upon TDP‐43 knockdown in neurons. Luciferase reporter assays and chromatin immunoprecipitation suggest that TDP‐43 represses VPS4B transcription. Preventing VPS4B upregulation or expression of its functional antagonist ALIX restores trafficking of recycling endosomes. Proteomic analysis revealed the broad reduction in surface expression of key receptors upon TDP‐43 knockdown, including ErbB4, the neuregulin 1 receptor. TDP‐43 knockdown delays the surface delivery of ErbB4. ErbB4 overexpression, but not neuregulin 1 stimulation, prevents dendrite loss upon TDP‐43 knockdown. Thus, impaired recycling of ErbB4 and other receptors to the cell surface may contribute to TDP‐43‐induced neurodegeneration by blocking trophic signaling.

Antibodies inhibit transmission and aggregation of C9orf72 poly‐GA dipeptide repeat proteins

Cell‐to‐cell transmission of protein aggregates is an emerging theme in neurodegenerative disease. Here, we analyze the dipeptide repeat (DPR) proteins that form neuronal inclusions in patients with hexanucleotide repeat expansion C9orf72, the most common known cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). Sense and antisense transcripts of the (G4C2)n repeat are translated by repeat‐associated non‐ATG (RAN) translation in all reading frames into five aggregating DPR proteins. We show that the hydrophobic DPR proteins poly‐GA, poly‐GP, and poly‐PA are transmitted between cells using co‐culture assays and cell extracts. Moreover, uptake or expression of poly‐GA induces nuclear RNA foci in (G4C2)80‐expressing cells and patient fibroblasts, suggesting an unexpected positive feedback loop. Exposure to recombinant poly‐GA and cerebellar extracts of C9orf72 patients increases repeat RNA levels and seeds aggregation of all DPR proteins in receiver cells expressing (G4C2)80. Treatment with anti‐GA antibodies inhibits intracellular poly‐GA aggregation and blocks the seeding activity of C9orf72 brain extracts. Poly‐GA‐directed immunotherapy may thus reduce DPR aggregation and disease progression in C9orf72 ALS/FTD.

Targeting RNA G‐quadruplexes as new treatment strategy for C9orf72 ALS/FTD

The recent discovery of a pathogenic expansion of a (GGGGCC)n repeat in the C9orf72 gene in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) led to a burst of mechanistic discoveries. In this issue, Simone et al ( ) describe novel compounds targeting the G‐quadruplex (G‐Q) structure of the (GGGGCC)n repeat RNA that alleviate the hallmarks of C9orf72 disease in patient‐derived neurons and increase survival in a Drosophila model. Lack of overt off‐target effects and toxicity suggest that these small molecules are promising lead compounds to the development of a therapy.

RNA versus protein toxicity in C9orf72 ALS/FTLD

A (G4C2)n expansion with several hundred or thousand repeats in the first intron upstream of the C9orf72 coding region is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD), but the driver mechanism remains unclear [7]. Three main pathomechanisms have been proposed, but their relative role is vigorously debated because they require partially opposing therapeutic strategies. Three reports in this issue take a provocative stance strongly arguing for either RNA or protein toxicity in C9orf72 pathogenesis (Fig. 1). Sense and antisense transcripts of the repeat accumulate in ubiquitous small nuclear, and occasionally cytoplasmic, RNA foci. Many (G4C2)n-binding proteins have been identified that are partially sequestered by the repeat RNA. Several of the trapped RNA-binding proteins are involved in alternative splicing and splicing abnormalities have been reported in C9orf72 patients [22, 25]. However, a sophisticated study on 63 C9orf72 cases shows no correlation of sense and antisense foci with neurodegeneration or clinical parameters [6], although antisense foci have been linked to TDP-43 pathology by others [5]. Repeat-associated non-ATG (RAN) translation of both sense and antisense repeat transcripts in all reading frames generates five co-aggregating dipeptide repeat (DPR) proteins: poly-GA/GP/GR from the sense transcript and polyGP/PA/PR from the antisense transcript. The sense-strand derived DPRs are abundant throughout the neocortex, hippocampus, thalamus and cerebellum, but scarce in brain stem and spinal cord. Although the DPR proteins co-aggregate predominantly in cytoplasmic and less frequently in intranuclear inclusions in neurons, the individual proteins have very different biophysical properties. Cryoelectron tomography shows that poly-GA forms twisted ribbons that interfere with proteasome function [9]. Poly-GR and -PR undergo liquid–liquid phase separation in vitro and interfere with the dynamics of the nucleolus and stress granules [13] and disturb nucleocytoplasmic transport [12]. The aggregate distribution of neither DPR species has been shown to correlate with TDP-43 pathology or neurodegeneration [16, 17, 27]. Finally, the repeat expansion interferes with transcription and/or splicing and leads to lower C9orf72 expression, a protein that has recently been linked to autophagy [8]. Homozygous knockout of C9orf72 causes a variable (lupuslike) immune phenotype due to strong expression of C9orf72 in the myeloid lineage, but the mice show no overt neurodegeneration [1, 20]. However, C9orf72 haploinsufficiency sensitizes patient-derived motoneurons to DPR toxicity and other stressors, suggesting it may contribute to neurodegeneration in C9orf72 ALS/FTD [28]. Thus, lowering C9orf72 expression to treat gain-of-function mechanisms may be detrimental. Swinnen et al. [29] injected zebrafish embryos with sense and antisense C9orf72 repeat RNA or synthetic genes encoding individual DPR proteins. This method is more rapid than transgenesis and allows dosage studies; however, only developing embryos can be analyzed. They focus their analysis on the length and branching pattern of motor axons during outgrowth. Consistent with severe toxicity in other models, (GR)50 and (PR)50 expression resulted in shorter axons with abnormal branching pattern, while the other individual DPRs had no effect. Expression of sense or antisense RNA had a similar effect starting at 35 or 70 repeats, respectively. In all cases, toxicity was dose dependent. Strikingly, repeat RNA injection did not result in significant DPR expression via RAN-translation in zebrafish embryos. Poly-GR/PR was not detectable by dot blot, although poly-GR/PR could still * Dieter Edbauer dieter.edbauer@dzne.de

A novel CHCHD10 mutation implicates a Mia40‐dependent mitochondrial import deficit in ALS

CHCHD10 mutations are linked to amyotrophic lateral sclerosis, but their mode of action is unclear. In a 29‐year‐old patient with rapid disease progression, we discovered a novel mutation (Q108P) in a conserved residue within the coiled‐coil‐helix‐coiled‐coil‐helix (CHCH) domain. The aggressive clinical phenotype prompted us to probe its pathogenicity. Unlike the wild‐type protein, mitochondrial import of CHCHD10 Q108P was blocked nearly completely resulting in diffuse cytoplasmic localization and reduced stability. Other CHCHD10 variants reported in patients showed impaired mitochondrial import (C122R) or clustering within mitochondria (especially G66V and E127K) often associated with reduced expression. Truncation experiments suggest mitochondrial import of CHCHD10 is mediated by the CHCH domain rather than the proposed N‐terminal mitochondrial targeting signal. Knockdown of Mia40, which introduces disulfide bonds into CHCH domain proteins, blocked mitochondrial import of CHCHD10. Overexpression of Mia40 rescued mitochondrial import of CHCHD10 Q108P by enhancing disulfide‐bond formation. Since reduction in CHCHD10 inhibits respiration, mutations in its CHCH domain may cause aggressive disease by impairing mitochondrial import. Our data suggest Mia40 upregulation as a potential therapeutic salvage pathway.