Summary An intronic GGGGCC repeat expansion in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), but its pathogenic mechanism remains unclear. Here we use human induced motor neurons (iMNs) to show that repeat-expanded C9ORF72 is haploinsufficient in ALS. We show that C9ORF72 interacts with endosomes and is required for normal vesicle trafficking and lysosomal biogenesis in motor neurons. Repeat expansion reduces 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 leads to neurodegeneration. Restoring C9ORF72 levels or augmenting its function with constitutively active RAB5 or chemical modulators of RAB5 effectors rescues patient neuron survival and ameliorates neurodegenerative processes in both gain- and loss-of function C9ORF72 mouse models. Thus, modulating vesicle trafficking can 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/FTD.

SummaryPrimary tauopathies are characterized neuropathologically by inclusions containing abnormal forms of the microtubule-associated protein tau (MAPT) and clinically by diverse neuropsychiatric, cognitive, and motor impairments. Autosomal dominant mutations in the MAPT gene cause heterogeneous forms of frontotemporal lobar degeneration with tauopathy (FTLD-Tau). Common and rare variants in the MAPT gene increase the risk for sporadic FTLD-Tau, including progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD). We generated a collection of fibroblasts from 140 MAPT mutation/risk variant carriers, PSP, CBD, and cognitively normal controls; 31 induced pluripotent stem cell (iPSC) lines from MAPT mutation carriers, non-carrier family members, and autopsy-confirmed PSP patients; 33 genome engineered iPSCs that were corrected or mutagenized; and forebrain neural progenitor cells (NPCs). Here, we present a resource of fibroblasts, iPSCs, and NPCs with comprehensive clinical histories that can be accessed by the scientific community for disease modeling and development of novel therapeutics for tauopathies.

IMPORTANCE Human motor neurons may be reliably derived from induced pluripotent stem cells (iPSCs). In vivo transplant studies of human iPSCs and their cellular derivatives are essential to gauging their clinical utility.OBJECTIVE To determine whether human iPSC-derived motor neurons can engraft in an immunodeficient mouse model of sciatic nerve injury. DESIGN, SETTING, AND SUBJECTSThis nonblinded interventional study with negative controls was performed at a biomedical research institute using an immunodeficient, transgenic mouse model. Induced pluripotent stem cell-derived motor neurons were cultured and differentiated. Cells were transplanted into 32 immunodeficient mice with sciatic nerve injury aged 6 to 15 weeks. Tissue analysis was performed at predetermined points after the mice were killed humanely. Animal experiments were performed from February 24, 2015, to May 2, 2016, and data were analyzed from April 7, 2015, to May 27, 2016.INTERVENTIONS Human iPSCs were used to derive motor neurons in vitro before transplant. MAIN OUTCOMES AND MEASURESEvidence of engraftment based on immunohistochemical analysis (primary outcome measure); evidence of neurite outgrowth and neuromuscular junction formation (secondary outcome measure); therapeutic effect based on wet muscle mass preservation and/or electrophysiological evidence of nerve and muscle function (exploratory end point).RESULTS In 13 of the 32 mice undergoing the experiment, human iPSC-derived motor neurons successfully engrafted and extended neurites to target denervated muscle. Human iPSC-derived motor neurons reduced denervation-induced muscular atrophy (mean [SD] muscle mass preservation, 54.2% [4.0%]) compared with negative controls (mean [SD] muscle mass preservation, 33.4% [2.3%]) (P = .04). No electrophysiological evidence of muscle recovery was found.CONCLUSIONS AND RELEVANCE Human iPSC-derived motor neurons may have future use in the treatment of peripheral motor nerve injury, including facial paralysis.LEVEL OF EVIDENCE NA.

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