Synaptic dysfunction in a hIPSC-derived neuronal model of ALS and FTD

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are adult-onset neurodegenerative disorders that share common pathology and genetic causes. In many cases the RNA binding protein TDP-43 aggregates in the cytoplasm leading to the loss of its nuclear functions, including as a splicing repressor. This results in the erroneous inclusion of intronic sequences known as cryptic exons (CEs) in several mature mRNAs. Recently, it has been shown that TDP-43 depletion leads to CE inclusion in UNC13A, which encodes a protein crucial for priming synaptic vesicles for release. The UNC13A CE was observed in neurons from ALS-FTD patients and leads to loss of both transcript and protein. In mouse models, loss of the homologous protein Munc13-1 leads to a severe impairment in glutamatergic neurotransmission. However, in human neurons the synaptic phenotype caused by loss of TDP-43, and subsequently Unc13A, has not been explored. Here, we used hIPSC-derived glutamatergic i3 neurons in which TDP-43 degradation can be induced as a model for ALS-FTD. To investigate the effect of TDP-43 loss on synaptic function we used electrophysiology and functional imaging of genetically encoded reporters including the glutamate sensor iGluSNFR3 and SypHy-RGECO, a dual reporter of calcium influx and vesicle fusion. We observed a decrease in the frequency of spontaneous release, deficits in evoked neurotransmitter release and changes in the dynamics of spontaneous network activity. We have also developed a rescue model in which the UNC13A CE is deleted, to confirm which of these changes are driven by loss of Unc13A.