To determine if C9ORF72 iMNs recapitulate neurodegenerative ALS processes, we examined their survival by performing longitudinal tracking of Hb9::RFP+ iMNs (Fig. 1a). This approach enabled us to distinguish differences in neurogenesis from differences in survival, which could not be addressed using previously-reported cross-sectional analyses6,7,10,26. In basal neuronal medium supplemented with neurotrophic factors, control and C9ORF72 patient iMNs survived equally well (Fig. 1b, Supplementary Fig. 3a, Supplementary Tables 5, 6). As human C9ORF72 ALS patients have elevated glutamate levels in their cerebrospinal fluid (possibly triggered by DPR-mediated aberrant splicing of the astrocytic excitatory amino acid transporter 2 EAAT2 4,27) we stimulated iMN cultures with a high glutamate pulse (12-hour treatment, 10 μM glutamate). This initiated a robust degenerative response in patient, but not control, iMNs (Fig. 1c-e and Supplementary Videos 3, 4) that was consistent across lines from multiple patients (n=6 patients) and controls (n=4 controls)(Fig. 1c, d and Supplementary Fig. 3d, e). While iMN survival varied slightly between live imaging systems, or between independent experiments due to the lengthy time course of neurodegeneration, the relative difference between control and C9-ALS patient iMNs was consistent (Fig. 1c - Nikon Biostation CT and Supplementary Fig. 3b - Molecular Devices ImageExpress). Moreover, iMNs from different iPSC lines derived from the same donor behaved similarly, suggesting genotypic differences accounted for these effects (Supplementary Fig. 3c). Treatment with glutamate receptor antagonists during glutamate administration prevented patient iMN degeneration (Fig. 1f). Alternatively, withdrawal of neurotrophic factors without glutamate stimulation also caused rapid degeneration of patient iMNs (n=3 patients, (Fig. 1g and Supplementary Fig. 3f).
Human EEA1 (1–209) with an N-terminal GST tag in pGEX-6P-1 vector or GST only were expressed in E. Coli BL21 (DE3) cells (Thermo Fisher Scientific) for 12 hr at 18℃. Harvested cells were lysed by sonication in cold GST Purification Buffer (50 mM Tris pH 8.0, 200 mM NaCl, 2 mM DTT, 0.5 mg/ml Lysozyme, 0.2% Triton X-100 and protease inhibitor cocktail (Roche)). After centrifugation at 15,000 g for 30 min at 4℃, clarified lysate was incubated with Glutathione Sepharose 4B beads (GE Healthcare Life Science) for 3 hours to purify GST-EEA1 or GST. HEK cells were transfected with C-terminal 3XFLAG tagged C9ORF72 isoform A or B, or eGFP constructs and harvested 36–48 hr post-transfection in cold Lysis Buffer (25 mM HEPES pH 7.4, 100 mM NACl, 5 mM MgCl2, 1 mM DTT, 10% Glycerol, 0.1% Triton X-100 and protease inhibitor cocktail (Roche)).After centrifugation at 8,000 g for 10 min at 4℃, the clarified supernatant was incubated with washed GST-EEA1 or GST beads for 2 hr at 4℃ with end-to-end rotation. Beads were then boiled in 2X SDS-PAGE sample buffer and pulled-down protein was analyzed by western blot.
We show for the first time that chemical or genetic modulators of vesicle trafficking can fully rescue iMN degeneration caused by the C9ORF72 repeat expansion. Previous studies have implicated several rare ALS or FTD mutations linked to these vesicle trafficking pathways, but by showing that C9ORF72 is haploinsufficient in ALS/FTD and demonstrating that perturbation of vesicle trafficking rescues C9ORF72 neurodegeneration, our findings highlight mechanistic convergence in a large portion of ALS.
Given our observation that iMNs with reduced C9ORF72 levels are hypersensitive to DPR toxicity, we wondered if this might be due to a general disruption of protein turnover by DPRsHowever, PR50-GFP expression did not impair turnover of APP or Tau (Supplementary Fig. 14f, g and Supplementary Fig. 5l). Thus the neurotoxicity caused by DPRs that accumulate rapidly in C9-ALS motor neurons due to reduced C9ORF72 levels is not due to global disruption of protein turnover.
(a) Production of Hb9::RFP+ iMNs and survival tracking by time-lapse microscopy. (b-d) Survival of control (CTRL) and C9ORF72 patient (C9-ALS) iMNs with neurotrophic factors (b) or in excess glutamate (shown with iMNs from all lines in aggregate (b, c) or for each individual line separately (d)). For (b-d), n=50 iMNs per line for 2 control and 3 C9-ALS lines, iMNs quantified from 3 biologically independent iMN conversions per line. (e) iMNs at day 22 in excess glutamate. This experiment was repeated three times with similar results. (f-g) Survival of control and C9-ALS iMNs in excess glutamate with glutamate receptor antagonists (f) or without neurotrophic factors (g). For (f-g), n=50 iMNs per line for 2 control and 3 C9-ALS lines, iMNs quantified from 3 biologically independent iMN conversions per line. (h) Survival of induced dopaminergic (iDA) neurons in excess glutamate. n=50 iMNs per line for 2 control and 2 C9-ALS lines, iMNs quantified from 3 biologically independent iMN conversions per line. Except in (d), each trace includes neurons from at least 2 donors with the specified genotype; see full detail in Methods. Scale bar: 100 μm (e). All iMN survival experiments were analyzed by two-sided log-rank test, and statistical significance was calculated using the entire survival time course. iMN survival experiments in (b-g) were performed in a Nikon Biostation and experiments in (h) were performed in a Molecular Devices ImageExpress.
Importantly, our work establishes a new approach for suppressing DPR protein toxicity and blocking C9ORF72 pathogenesis: restoring or replacing C9ORF72 activity. Although high levels of C9ORF72 isoform A may have slightly detrimental effects on control motor neuron survival, we have only observed this in neurons without C9ORF72 repeat expansion. Thus, we would not anticipate a harmful effect of forced C9ORF72 expression in C9ORF72 patients. In addition, a better understanding of the effects of forced C9ORF72 expression could inform safe development of this therapeutic strategy. For example, determining if C9ORF72 accelerates turnover of DPR aggregates by stimulating autophagy could lead help to identify new therapeutic targets.
HEK 293T cells were used to produce retrovirus, lentivirus, and C9ORF72 protein. HEK cells were used for these purposes based on previous published studies using HEK cells in order to produce viral particles and mammalian proteins. HEK cells were obtained from American Type Culture Collection, catalog number CRL-11268. HEK and iPS cells were tested for mycoplasma before, during, and after the study and were negative.
To determine if reduced C9orf72 levels leads to glutamate receptor accumulation in vivo, we examined spinal motor neurons deleted of C9orf72 in Nestin-Cre-Stop-Flox-C9orf72 mice 22. Immunofluorescence analysis indicated that Nr1 (NMDA) and GluR1 (AMPA) levels were elevated in C9orf72-null motor neurons (Supplementary Fig. 12a, b). To confirm these findings, we isolated post-synaptic densities from the spinal cords of control and C9orf72 knockout mice. Post-synaptic density fractions contained glutamate receptors and PSD-95, but not p53 or synaptophysin, indicating they were enriched for post-synaptic density proteins (Supplementary Fig. 12c, 5i). Immunoblotting showed that post-synaptic densities in C9orf72 knockout mice contained significantly higher levels of Nr1 and Glur1 than in control mice (Fig. 4i, j and Supplementary Fig. 5j).
An intronic GGGGCC repeat expansion in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but the pathogenic mechanism of this repeat remains unclear. Using human induced motor neurons (iMNs), we found that repeat-expanded C9ORF72 was haploinsufficient in ALS. We found that C9ORF72 interacted with endosomes and was required for normal vesicle trafficking and lysosomal biogenesis in motor neurons. Repeat expansion reduced C9ORF72 expression, triggering neurodegeneration through two mechanisms: accumulation of glutamate receptors, leading to excitotoxicity, and impaired clearance of neurotoxic dipeptide repeat proteins derived from the repeat expansion. Thus, cooperativity between gain- and loss-of-function mechanisms led to neurodegeneration. Restoring C9ORF72 levels or augmenting its function with constitutively active RAB5 or chemical modulators of RAB5 effectors rescued patient neuron survival and ameliorated neurodegenerative processes in both gain- and loss-of-function C9ORF72 mouse models. Thus, modulating vesicle trafficking was able to rescue neurodegeneration caused by the C9ORF72 repeat expansion. Coupled with rare mutations in ALS2, FIG4, CHMP2B, OPTN and SQSTM1, our results reveal mechanistic convergence on vesicle trafficking in ALS and FTD.
A 241-bp digoxigenin (DIG)-labeled probe was generated from 100 ng control genomic DNA (gDNA) by PCR reaction using Q5® High-Fidelity DNA Polymerase (NEB) with primers shown in Supplementary Data Table 4. Genomic DNA was harvested from control and patient iPSCs using cell lysis buffer (100 mM Tris-HCl pH 8.0, 50 mM EDTA, 1% w/v sodium dodecyl sulfate (SDS)) at 55ºC overnight and performing phenol:chloroform extraction. A total of 25 µg of gDNA was digested with XbaI at 37 ºC overnight, run on a 0.8% agarose gel, then transferred to a positive charged nylon membrane (Roche) using suction by vacuum and UV-crosslinked at 120 mJ. The membrane was pre-hybridized in 25 ml DIG EasyHyb solution (Roche) for 3 h at 47 ºC then hybridized at 47 ºC overnight in a shaking incubator, followed by two 5-min washes each in 2X Standard Sodium Citrate (SSC) and in 0.1% SDS at room temperature, and two 15-min washes in 0.1x SSC and in 0.1% SDS at 68 ºC. Detection of the hybridized probe DNA was carried out as described in DIG System User’s Guide. CDP-Star® Chemilumnescent Substrate (Sigma-Aldrich) was used for detection and the signal was developed on X-ray film (Genesee Scientific) after 20 to 40 min.
Removal of TTX and TEA during glutamate receptor agonist treatment revealed additional increases in Gcamp6 activation in C9ORF72+/− iMNs compared to controls, suggesting that C9ORF72+/− iMNs also fire action potentials more frequently than controls (Supplementary Fig. 13a), although we did not detect large changes in sodium or potassium current amplitudes in C9ORF72+/− iMNs (Supplementary Fig. 13b, c). To determine if increased neuronal activity due in part to elevated glutamate receptor levels contributes to neurodegeneration in C9ORF72 patient and C9ORF72+/− iMNs, we measured iMN survival in the presence or absence of retigabine. Retigabine is approved by the U.S. Food and Drug Administration for the treatment of epilepsy and reduces neuronal excitability by activating Kv7 potassium channels 48. In the glutamate treatment assay, retigabine increased the survival of C9ORF72 patient (n=2 patients) and C9ORF72-deficient iMNs, but not controls (n=2 controls)(Supplementary Fig. 13d-g).
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GCaMP6 was cloned into the pMXs-Dest-WRE retroviral vector and transduced into reprogramming cultures concurrently with the motor neuron factors. To assess GCaMP6 activity, 1.5 μm glutamate was added to iMN cultures and cells were imaged continuously for 2 minutes at 24 frames per second. GFP flashes were scored manually using the video recording. At least 3 different fields of view from three independent cultures, totalling 50–100 iMNs, were scored per condition.
Complementary DNAs (cDNAs) for the iMN factors (Ngn2, Lhx3, Isl1, NeuroD1, Ascl1, Myt1l, and Brn2) and iDA neuron factors (Ascl1, Brn2, Myt1l, Lmx1a, and Foxa2), were purchased from Addgene. cDNA for C9ORF72 was purchased from Thermo Scientific. Each cDNA was cloned into the pMXs retroviral expression vector using Gateway cloning technology (Invitrogen). The Hb9::RFP lentiviral vector was also purchased from Addgene (ID: 37081). Viruses were produced as follows. HEK293 cells were transfected at 80–90% confluency with viral vectors containing genes of interest and viral packaging plasmids (PIK-MLV-gp and pHDM for retrovirus; pPAX2 and VSVG for lentivirus) using polyethylenimine (PEI)(Sigma-Aldrich). The medium was changed 24h after transfection. Viruses were harvested at 48h and 72 h after transfection. Viral supernatants were filtered with 0.45 µM filters, incubated with Lenti-X concentrator (Clontech) for 24 h at 4 ºC, and centrifuged at 1,500 x g at 4ºC for 45 min. The pellets were resuspended in 300 µl DMEM + 10% FBS and stored at −80 ºC.
Postsynaptic density extraction was done following a protocol published previously 63. Briefly, mouse spinal cord tissue or human cortical tissue was homogenized in cold Sucrose Buffer (320 mM Sucrose, 10 mM HEPES pH 7.4, 2 mM EDTA, 30 mM NaF, 40 mM β-Glycerophosphate, 10 mM Na3VO4, and protease inhibitor cocktail (Roche)) using a tissue grinder and then spun down at 500 g for 6 min at 4℃. The supernatant was re-centrifuged at 10,000 g for 10 min at 4℃. The supernatant was collected as the “Total” fraction, and the pellet was resuspended in cold Triton buffer (50 mM HEPES pH 7.4, 2 mM EDTA, 50 mM NaF, 40 mM β-Glycerophosphate, 10 mM Na3VO4, 1% Triton X-100 and protease inhibitor cocktail (Roche)) and then spun down at 30,000 RPM using a Beckman rotor MLA-130 for 40 min at 4℃. The supernantant was collected as the “Triton” fraction and the pellet was resuspended in DOC buffer (50 mM HEPES pH 9.0, 50 mM NaF, 40 mM β-Glycerophosphate, 10 mM Na3VO4, 20 uM ZnCl2, 1% Sodium Deoxycholate and protease inhibitor cocktail (Roche)) and collected as the “DOC”, PSD-enriched fraction. Collected samples were boiled with SDS-PAGE sample buffer and analyzed by western blot. Purity of the PSD preps was analyzed by immunoblotting for PSD-95 (PSD), p53 (non-PSD), and synaptophysin (non-PSD).
Samples were first fixed in 4% PFA (1x PBS) overnight at 4°C and were subsequently washed three times with 1x PBS. Next, cells were permeabilized with 0.3% Triton X-100 (1x PBS) for 10 min at room temperature, followed by three washes with 1x PBS for 10 min each. After permeabilization, the samples were equilibrated in 1x SSC buffer for 10 min at room temperature and then transferred into 50% formamide (2x SSC) for 10 min at 60°C. The repeat expansion-targeting probe and the negative control probe were ordered from Exiqon 58. During this step, the probe mixture (1µl salmon sperm (10 µg/µl), 0.5 µl E. coli tRNA (20 µg/µl), 0.4 µl probe (25 µM), 25 µl 80% formamide/per sample) was made and placed at 95°C for at least 10 min. The samples were submerged in 200 µl of hybridization buffer (4ml 100% formamide, 0.5 ml 20x SSC, 1 ml BSA fraction V, 0.5ml RVC (20 mM), 1ml NaPO4 (0.1 M), 3 ml nuclease-free water) and 27 µl of the probe mixture was added to each sample and incubated for 1 hour at 60°C. After probe hybridization, the samples were washed twice with 50% formamide (2x SSC) for 20 min each at 65°C and once more with 40% formamide (1x SSC) for 10 min at 60°C. The remaining formamide was removed by briefly washing with 1x SSC three times. A final crosslinking step was performed by first incubating the samples with 1x Tris-Glycine for 5 minutes followed by a 5 min incubation in 4% PFA. Samples were washed three times with 1x PBS, stained with DAPI, and imaged using a Zeiss LSM 800 confocal microscope.
Live imaging of iMNs expressing a M6PR-GFP fusion protein that localizes to M6PR+ vesicles 44 confirmed that C9ORF72 patient and C9ORF72-deficient iMNs possess increased numbers of M6PR+ vesicle clusters, and that overexpression of C9ORF72 isoform A or B rescues this phenotype (Supplementary Fig. 9c-g and Supplementary Videos 5-9). Clusters did not disperse over the time course of the assay, suggesting that they are relatively stable and not in rapid flux (Supplementary Videos 5-9). In addition, M6PR+ puncta moved with a slower average speed in C9ORF72 patient and C9ORF72+/− iMNs than controls (Supplementary Fig. 9h, i). Thus, reduced C9ORF72 levels lead to fewer lysosomes in motor neurons in vitro and in vivo, and this may be due in part to altered trafficking of M6PR+ vesicles.
Previously described Nestin‐Cre+/− C9orf72loxP/loxP mice22 (18 months old) and age-matched controls (n=2 per genotype, 1 male and 1 female for C9orf72−/− and 2 males for C9orf72+/+) were transcardially perfused with phosphate buffered saline (PBS) and subsequently with 4% formaldehyde. Cryoprotection occurred in 30% sucrose. Additionally, 6 month old C9-BAC and wildtype controls were transcardially perfused with phosphate buffered saline (PBS) and subsequently with 4% formaldehyde. Cryoprotection occurred in 20% sucrose. After snap freezing, tissue was sectioned by cryostat at 20 µm thickness and stained with the following primary antibodies: NMDAR1, GluR1, GluR6/7, Lamp1, NeuN, Chat, and GR. Antigen retrieval by sodium citrate occurred prior to GR staining. Images were collected using a Zeiss LSM780 or LSM800 confocal microscope. Glutamate subunit intensity measurements occurred with ImageJ where the mean cytosolic intensity was divided by a background measurement collected near to the measured neuron. Lamp1 and GR quantification occurred with the help of ImageJ. The scientist performing the glutamate receptor subunit intensity, LAMP1 vesicle, and GR quantification was blinded to the genotypes or treatment of the samples.

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Amongst four reproducible hit compounds, we identified a PIKFYVE kinase inhibitor (YM201636) that significantly increased C9ORF72 patient iMN survival (n=2 patients) (Fig. 6b, c and Supplementary Fig. 15a). PIKFYVE is a lipid kinase that converts phosphatidylinositol 3-phosphate (PI3P) into phosphtidylinositol (3,5)-bisphosphate (PI(3,5)P2)51(Fig. 6f). PI3P is primarily generated by PI3-kinases recruited to early endosomes by RAB5, and PI3P anchors EEA1 to early endosomes to drive endosomal maturation 52(Fig 6f). Following endosomal maturation into lysosomes, PI3P drives fusion of lysosomes with autophagosomes 53. PIKFYVE regulates PI3P levels by converting PI3P into PI(3,5)P2 52, which disfavors lysosomal fusion with endosomes and autophagosomes 53,54. Therefore, inhibition of PIKFYVEincreases autophagosome-lysosome fusion 53 and may compensate for reduced C9ORF72 activity and other disease processes by increasing PI3P levels to facilitate removal of glutamate receptors or DPRs (Fig. 6f). Interestingly, FIG4 is a phosphatase that opposes PIKFYVE kinase by converting PI(3,5)P2 to PI3P and loss-of-function mutations in FIG4 cause ALS 55. Thus, genetic evidence suggests that PIKFYVE inhibition may be capable of modulating ALS disease processes in humans.
To generate Dox-NIL iMNs, the Dox-NIL construct was integrated into the AAVS1 safe harbor locus of the control, C9+/−, and C9-ALS patient iPSC lines using CRISPR/Cas9 editing (gRNA sequence shown in Supplementary Table 4). Dox-NIL iMNs were generated by plating at ~25% confluency on matrigel coated plates and adding 1 μg/ml of doxycylin in N3 media +7.5 μM RepSox 1 day after plating. Mouse primary mixed glia were added to the cultures at day 6, and doxycyline was maintained throughout conversion. iMN cultures were harvested at day 17.

The following antibodies were used in this manuscript: mouse anti-HB9 (Developmental Studies Hybridoma Bank); 81.5C10. chicken anti-TUJ1 (EMD Millipore); AB9354. rabbit anti-VACHT (Sigma); SAB4200559. rabbit anti-C9ORF72 (Sigma-Aldrich); HPA023873. rabbit anti-C9ORF72 (Proteintech); 25757–1-AP. mouse anti-EEA1 (BD Biosciences); 610457. mouse antiRAB5 (BD Biosciences); 610281. mouse anti-RAB7 (GeneTex); GTX16196. mouse anti-LAMP1 (Abcam); ab25630. mouse anti-M6PR (Abcam); ab2733. rabbit anti-GluR1 (EMD Millipore); pc246. mouse anti-NR1 (EMD Millipore); MAB363. chicken anti-GFP (GeneTex); GTX13970. rabbit anti-Glur6/7 (EMD Millipore); 04–921. mouse anti-FLAG (Sigma); F1804. mouse anti-GAPDH (Santa Cruz); sc-32233. chicken anti-MAP2 (Abcam); ab11267, rabbit anti-GLUR1 (Millipore, cat. no. 1504), mouse anti-NR1 (Novus, cat. no. NB300118), mouse anti-Transferrin receptor (Thermo, cat. no. 136800), mouse anti-LAMP3 (DSHB, cat. no. H5C6), rabbit anti-LAMP3 (Proteintech, cat. no. 12632), mouse anti-LAMP2 (DSHB, cat. no. H4B4), goat anti-HRP (Santa Cruz, cat. no. sc-47778 HRP), mouse anti-TUJ1 (Biolegend, cat. no. MMS-435P), rabbit anti-APP (Abcam, cat. no. ab32136), mouse anti-Tau5 (Thermo, cat. no. AHB0042), mouse anti-PSD-95 (Thermo, cat. no. MA1–045), mouse anti-p53 (Cell Signaling, cat. no. 2524S), anti-mouse HRP (Cell Signaling, cat. no. 7076S), anti-rabbit HRP (Cell Signaling, cat. no. 7074S).
To determine if the survival difference between C9ORF72 patient iMNs and controls was specific to our transcription factor-based reprogramming approach, we also measured the survival of Hb9::RFP+ control and C9ORF72 patient motor neurons derived from iPSCs by small molecule activation of the Sonic Hedgehog and retinoic acid signaling pathways 28 (Supplementary Fig. 3g, h). Similarly to iMNs, morphogen-generated motor neurons showed a significant survival difference between C9ORF72 patients and controls (Supplementary Fig. 3i-l).
We compared the differential expression results from our data to other transcriptomic datasets in ALS, obtained from the Gene Expression Omnibus (GEO). Raw Affymetrix array data (.CEL files) were downloaded for dataset GSE56504, and preprocessed using a standard exon array pipeline implemented using the R Bioconductor package oligo. For GSE56504, only the laser-capture microdissection samples were included/ Differential expression was calculated using the R Bioconductor package limma. RNA-seq counts data was obtained for dataset GSE67196. For GSE67196, only the frontal cortex samples were included. Normalization and differential expression analysis were performed using DESeq2.

Immunostaining revealed that C9ORF72+/− and C9ORF72−/− iMNs contained elevated levels of NMDA (NR1) and AMPA (GLUR1) receptors on neurites and dendritic spines compared to control iMNs under basal conditions (Fig. 4a, c, d and Supplementary Fig. 5b and 10a, c-e, g, h, j, k). In addition, control iMNs treated with C9ORF72-specific ASOs displayed increased numbers of NMDA and AMPA receptors in their neurites (Supplementary Fig. 10l, m). C9ORF72 patient iMNs (n=3 patients) also showed elevated NR1 and GLUR1 levels compared to controls (n=3 controls), and forced expression of C9ORF72 isoform B reduced glutamate receptor levels in patient iMNs (n=3 patients) to that of controls (n=3 controls) (Fig. 4a-c and Supplementary Fig. 10a-h). mRNA levels of NR1 (GRIN1) and GLUR1 (GRIA1) were not elevated in flow-purified C9ORF72+/− iMNs, indicating that increased transcription could not explain the increased glutamate receptor levels (Supplementary Fig. 10n).