Frontotemporal dementia (FTD) is an age-related non-Alzheimer dementia characterized by progressiveneuronal loss in the frontotemporal lobes. A subset of FTD is defined by the pathology of protein inclusionspositive for Fused in Sarcoma (FUS) thus named FUS-FTD. FTD and amyotrophic lateral sclerosis (ALS) sharea wide spectrum of clinical pathological and genetic features. Pathogenic mutations of FUS cause both ALS andFTD and FUS proteinopathy is also detected in sporadic diseases. FUS is primarily in the nucleus but theprotein with a disease-causing mutation mislocalizes and accumulates in the cytoplasm. Protein aggregatesinduced by FUS mutations may sequester other proteins to compromise cellular functions resulting in impairedneurons. We recently showed that FTD/ALS mutations of FUS suppressed protein translation and hyper-activated the nonsense-mediated decay (NMD) of mRNAs. The hypothesis to be tested in this project is that thedysregulation of protein translation and mRNA surveillance contributes to cortical neuron loss in FUS-FTD. Three specific aims are designed to test the hypothesis using in vitro and animal models as well as FTDpatient tissues. Aim 1 is to determine how mRNA NMD and protein translation are perturbed by pathogeneic FUSmutations in FTD mouse models and patient tissues. We will determine whether NMD factors and translation-related proteins are sequestered in FUS inclusions in forebrain neurons in R521G FUS transgenic mice atdifferent ages. We will measure mRNA turnover rates and protein translation efficiency in R521G FUS mice andcorrelate the perturbations to neuronal dysfunction and FTD disease progression. Moreover we will examinewhether NMD factors and protein translation proteins are sequestered in FTD patient tissues. Aim 2 is to identifyspecific proteins suppressed by pathogenic FUS in FTD mice. We will apply the puromycin labeling in FTD miceand use the proteomic approach to identify changes in protein translation impacted by mutant FUS in forebrainneurons. In addition actively translated mRNAs will be identified and quantified in polysome fractions using RNA-Seq. Results from the omics approaches will be integrated for pathway analysis to reveal whether pathogenicFUS impairs proteins in specific pathways providing novel insights into the FTD etiology. Aim 3 is to elucidate thesignificance of RNA binding and post-translational modifications in the dysfunction of pathogenic FUS. We willuse a cohort of RNA binding-deficient mutations in an optogenetic Cry2olig-FUS-mCherry system to examinethe significance of RNA binding in FUS inclusion formation and dysfunction in cortical neurons. We recentlyfound FUS is acetylated at residues critical to RNA binding we will test how acetylation-null and -mimickingmutations affect FUS inclusions NMD and protein translation. In addition we will also examine whetherpathogenic FUS forms RNA-dependent and -independent inclusions that produce different levels of toxicity toneurons. The proposed experiments will thoroughly examine a novel disease mechanism using innovativeapproaches. Completion of our proposed work will help elucidate molecular mechanisms underlying FUS FTD.