To determine if a deletion of C9ORF72 or the C9ORF72 repeat expansion caused changes in endosomal trafficking in motor neurons, we examined the number of early endosomes (RAB5+, EEA1+), late endosomes (RAB7+), and lysosomes (LAMP1+, LAMP2+, LAMP3+) in control, C9ORF72 patient, C9ORF72+/−, and C9ORF72−/− iMNs. We observed the most significant difference in the lysosomal population, with C9ORF72 patient iMNs (n=4 patients) having fewer LAMP1+, LAMP2+, and LAMP3+ vesicles than control iMNs (n=4 controls)(Fig. 3c, d and Supplementary Fig. 8a-d). C9ORF72+/− and C9ORF72−/− also harbored fewer LAMP1+, LAMP2+, and LAMP3+ vesicles than isogenic control iMNs, indicating that reduced C9ORF72 levels alone leads to a loss of lysosomes (Fig. 3c, e, f and Supplementary Fig. 8a-d). ASO-mediated knockdown of C9ORF72 expression also decreased lysosome numbers in iMNs (Supplementary Fig. 8e). Although membrane fractionation showed that control and patient iMNs have similar amounts of LAMP2 in the lysosomal membrane fraction (Supplementary Fig. 8f), analysis of the immunofluorescence intensity of LAMP proteins suggests that this is likely due to the fact that C9ORF72 patient and C9ORF72+/− iMNs have a higher concentration of LAMP proteins in their lysosomal membranes, possibly as a result of fewer lysosomes being present (Supplementary Fig. 8g). Using electron microscopy to identify lysosomes by their high election density 40, we verified that the vesicles reduced in C9ORF72-deficient cells were lysosomes (Fig. 3g-i). Forced expression of either C9ORF72 isoform restored the number of LAMP1+, LAMP2+, and LAMP3+ lysosomes in patient (n=4 patients) and C9ORF72-deficient iMNs (Fig. 3c-f and Supplementary Fig. 8a-h). To determine if loss of C9ORF72 activity reduces lysosome numbers in motor neurons in vivo, we measured the number of lysosomes in spinal motor neurons in Nestin-Cre-Stop-Flox-C9orf72 mice 22. C9orf72−/− motor neurons contained significantly fewer Lamp1+ lysosomes than control motor neurons (Fig. 3j, k).
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).
IPSC-MNs at differentiation D35 were harvested in cold Hypotonic buffer (20 mM HEPES pH 7.4, 10 mM KCl, 2 mM MgCl2, 1 mM EDTA, 1mM EGTA, 1 mM DTT and protease inhibitor cocktail (Roche)) and lysed by passing through G25 needles 25 times and then spun down at 700 x g for 10min at 4℃. The Supernatant was loaded onto pre-made 30% Percoll solution and re-centrifuged at 33,000 RPM using Beckman rotor SWI55 for 50min at 4℃. 300 ul aliquots were taken from top to bottom as fractions and all the collected samples were boiled with SDS-PAGE sample buffer and analyzed by western blot.
To determine if patient iMN degeneration resulted from bona fide ALS disease processes specific for motor neurons, we measured the survival of induced dopaminergic neurons (iDAs) generated by expression of FoxA2, Lmx1a, Brn2, Ascl1, and Myt1l 29. These neurons expressed high levels of tyrosine hydroxylase, indicating they had established a key aspect of the dopamine synthesis pathway and were distinct from iMNs, which do not express this enzyme 24 (Supplementary Fig. 3m, n). Unlike iMN cultures, iDA cultures from C9ORF72 patients (n=2 patients) did not show reduced survival compared to controls (n=2 controls) in either glutamate treatment and neurotrophic factor withdrawal conditions (Fig. 1h and Supplementary Fig. 3o), indicating that the in vitro neurodegenerative phenotype elicited by the C9ORF72 mutation is selective for motor neurons.
Our results indicate that haploinsufficiency for C9ORF72 activity triggers neurodegeneration in C9ORF72 ALS, and this occurs by at least two mechanisms. First, reduced C9ORF72 activity causes the accumulation of glutamate receptors and excitotoxicity in response to glutamate. Although C9orf72 knockout mice do not display overt neurodegeneration14,18,22, these mice may be protected from excitotoxicity because they lack gain-of-function disease processes such as DPRs, which induce aberrant splicing and dysfunction of the EAAT2 glutamate transporter in astrocytes in vitro 12 and in C9ORF72 ALS patients 4,27. EAAT2 dysfunction causes glutamate accumulation in the cerebrospinal fluid of ALS patients 27, and consistent with this notion, we found that poly(PR) expression in human astrocytes reduced their rate of glutamate uptake. By using human iMNs, mice, and human post mortem tissue, we show for the first time that reduced C9ORF72 activity modulates the vulnerability of human motor neurons to degenerative stimuli and establish a mechanistic link between the C9ORF72 repeat expansion and glutamate-induced excitotoxicity
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).