Impairment of Mitochondrial Calcium Buffering Links Mutations in C9ORF72 and TARDBP in iPS-Derived Motor Neurons from Patients with ALS/FTD

TDP-43 dysfunction is common to 97% of amyotrophic lateral sclerosis (ALS) cases, including those with mutations in C9orf72. To investigate how C9ORF72 mutations drive cellular pathology in ALS and to identify convergent mechanisms between C9ORF72 and TARDBP mutations, we analyzed motor neurons (MNs) derived from induced pluripotent stem cells (iPSCs) from patients with ALS. C9ORF72 iPSC-MNs have higher Ca2+ release after depolarization, delayed recovery to baseline after glutamate stimulation, and lower levels of calbindin compared with CRISPR/Cas9 genome-edited controls. TARDBP iPS-derived MNs show high glutamate-induced Ca2+ release. We identify here, by RNA sequencing, that both C9ORF72 and TARDBP iPSC-MNs have upregulation of Ca2+-permeable AMPA and NMDA subunits and impairment of mitochondrial Ca2+ buffering due to an imbalance of MICU1 and MICU2 on the mitochondrial Ca2+ uniporter, indicating that impaired mitochondrial Ca2+ uptake contributes to glutamate excitotoxicity and is a shared feature of MNs with C9ORF72 or TARDBP mutations. [Display omitted] •Mutations in C9orf72 and TDP-43 reduce Ca2+ buffering capacity in ALS motor neurons•High levels of Ca2+-permeable glutamate receptors link C9orf72 and TDP-43•Low mitochondrial Ca2+ uptake leads to glutamate-induced cell death•Imbalance of MICU1 and MICU2 reduces mitochondrial Ca2+ uptake In this study, Dafinca, Talbot, and colleagues provide a mechanistic link between mutations in C9orf72 and TARDBP in induced pluripotent stem cell-derived motor neurons from ALS patients. They show both mutations cause increased expression of Ca2+-permeable receptors, reduced cytosolic Ca2+ buffering, and identify that perturbations in mitochondrial calcium uptake through an imbalance of MICU1 and MICU2 link the mutations.