The overall objective of this proposal is using mouse models to examine the pathogenic mechanism by which loss of TAR DNA binding protein 43kDa (TDP-43) contributes to aberrant neural activity and, ultimately, neurodegeneration. TDP-43 predominantly functions as a nuclear protein important for transcription regulation. However, its cytosolic aggregates are commonly found in patients with neurodegenerative diseases including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Since cytosolic accumulation of TDP-43 is usually accompanied by its nuclear clearance, a loss-of-function mechanism has been proposed to contribute to neurodegeneration. The underlying pathogenic pathways driven by the loss-of-function of TDP-43 are largely unknown. Our preliminary in vivo calcium imaging studies demonstrated biphasic changes at population levels from pyramidal neurons of prefrontal cortex in spontaneously occurring calcium transients following TDP-43 depletion. We propose to study the pathogenic mechanisms of TDP-43 loss-of-function on two levels: Frist, we will repetitively measure from the same pyramidal neurons, the spontaneously occurring calcium transients over time, to elucidate phasic changes in neural calcium transients following TDP-43 depletion. Second, we will measure phasic changes in intrinsic excitability of pyramidal neurons elicited by TDP-43 depletion, to decipher the correlation between abnormalities in calcium transients and neural excitability. Completion of the proposed studies will determine how TDP-43 loss-of-function drives aberrant neural activity prior to neurodegeneration, whether hyperactivity and hypoactivity are mechanistically related, and which of the two is the primary driving force for pathogenesis. Filling these knowledge gaps will help to pave the new way towards early intervention for ALS and FTD.