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.
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.
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).

To verify that PIKFYVE is the functional target of the inhibitor, we first confirmed PIKFYVE expression by qPCR in control and patient (n=3 patients) iMNs (Supplementary Fig. 15b). Next, we verified that YM201636 rescued C9ORF72 patient iMN survival in a dose-dependent manner (Supplementary Fig. 15c). We then asked if Apilimod, a structurally distinct PIKFYVE inhibitor, could rescue patient iMN survival 51(Fig. 6b). To verify target engagement by Apilimod in iPSC-derived motor neurons, we administered Apilimod for three hours and measured EEA1+ early endosome size. PIKFYVE inhibition increases PI3P levels, leading to increased recruitment of EEA1 to early endosomes, more homotypic early endosomal fusion, and larger EEA1+ early endosomes 54. As expected, Apilimod treatment increased EEA1+ endosome size in a dose-dependent manner, verifying target engagement in motor neurons (Supplementary Fig. 15d, e).
Therapeutic strategies in development for C9ORF72 ALS/FTD target gain-of-function mechanisms. These include ASOs 6–8 and small molecules 13 that disrupt RNA foci formation. However, these approaches have not fully rescued neurodegeneration in human patient-derived neurons 6–8,13, indicating that replacing C9ORF72 function or new therapeutic targets may be required.

Structured illumination microscopy (SIM) images were acquired using a Zeiss Elyra PS.1 system equipped with a 100X 1.46 NA or 63X 1.4NA objective. Acquisition was performed with PCO edge sCMOS camera and image reconstruction was done with built-in structured illumination model. Confocal microscopy images were acquired using Zeiss LSM800 microcopy with 63X 1.4NA objective or Zeiss LSM780 microcopy with 40X 1.1NA objective. Z stack images were done with a step size of 2.5 um. Further image process was done with Fiji.
We anticipate three key implications of our findings: 1) ALS/FTD caused by the C9ORF72 repeat expansion requires both gain- and loss-of-function mechanisms, 2) increasing C9ORF72 activity in motor neurons should mitigate disease and provides a new therapeutic target, and 3) PIKFYVE inhibition and other approaches that modulate vesicle trafficking may ameliorate C9ORF72 disease processes in both neurons and myeloid cells. The fact that mutations in FIG4 cause ALS, epilepsy, and Charcot-Marie-Tooth 55 illustrates the broad implications of impaired vesicle trafficking within the CNS. The identification of targets that effectively modulate vesicle trafficking in neurons, glia, and myeloid cells could hold tremendous therapeutic value for C9ORF72 ALS/FTD and other CNS disorders.
International Advisory Board: James Archibald (Translation Studies) - Hugo de Burgh (Chinese Media Studies) - Kristen Brustad (Arabic Linguistics) - Daniel Coste (French Language) - Luciano Curreri (Italian Literature) - Claudio Di Meola (German Linguistics) - Donatella Dolcini (Hindi Studies) - Johann Drumbl (German Linguistics) - Denis Ferraris (Italian Literature) - Lawrence Grossberg (Cultural Studies) - Stephen Gundle (Film and Television Studies) - Tsuchiya Junji (Sociology) - John McLeod (Post-colonial Studies) - Estrella Montolío Durán (Spanish Language) - Silvia Morgana (Italian Linguistics) - Samir Marzouki (Translation, Cultural Relations) - Mbare Ngom (Post-Colonial Literatures) - Christiane Nord (Translation Studies) - Roberto Perin (History) - Giovanni Rovere (Italian Linguistics) - Lara Ryazanova-Clarke (Russian Studies) - Shi-Xu (Discourse and Cultural Studies) - Srikant Sarangi (Discourse analysis) - Françoise Sabban, Centre d'études sur la Chine moderne et contemporaine (Chinese Studies) - Itala Vivan (Cultural Studies, Museum Studies)
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).
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