The goal of the project is to understand the dysregulation of inhibitory synapse caused by poly-GR in C9ORF72-linked amyotrophic lateral sclerosis (ALS) and frontotemporal degeneration (FTD). Hexanucleotide repeat expansion in the C9ORF72 gene is the most common genetic cause of both ALS and FTD. One potential pathogenic mechanism is the aberrant accumulation of dipeptide repeat (DPR) proteins produced by repeat-associated non-AUG (RAN) translation in all six reading frames (poly-GA, poly-GR, poly-PA, poly-PR and poly-PG) of both sense and antisense RNAs. Lines of evidence indicate that some forms of the DPR proteins, particularly GR and PR, can induce dysregulation of several molecular pathways, including stress granule dysfunction, nucleocytoplasmic transport defects, altered ER homeostasis, et al. How these molecular deficits influence neuronal functions remains understudied. Accumulating evidence indicated that changes of synapse density and function occurs prior to significant neuronal death in neurodegenerative diseases. The development of hyperexcitability is a well-known phenomenon. One plausible mechanism is the alterations in the ratio of excitatory to inhibitory activity (E/I imbalance). Our preliminary data identified specific reduction of gephyrin clustering, a scaffold protein essential for inhibitory synapse formation and function, in C9ORF72-ALS/FTD patient iPSC-neurons and in primary neurons with poly-GR expression. We now propose to characterize the structure and electrophysiological dysfunction of the inhibitory synapse, and to decipher the molecular mechanism how poly-GR causes such defects by combining different –omics techniques with molecular and cellular approaches. We aim to identify GR-interacting proteins in neurons using the proximity-labeling proteomics and GR-induced transcriptomic changes by high-throughput sequencing, which will reveal candidate genes/pathways for further mechanistic study of their contribution to GR-mediated synaptic dysregulation. This project makes an important first step toward understanding the molecular mechanisms of neuronal dysfunction in ALS and FTD patients. The molecular insights resulting from this study will help understanding the etiology of the disease and developing novel therapeutic targets.