PROJECT SUMMARY Two-thirds of lysosomal storage disorders (LSD) affect the brain, yet most LSD treatments do not improve central nervous system (CNS) symptoms. Several trials of brain-directed gene therapy have failed to show clinical benefit despite restoring protein expression in the CNS. Outcomes are especially poor in subjects who have developed neurologic deficits, suggesting rescue of expression alone may be insufficient to correct function once diseased neuronal circuits are established. In CLN3 Disease, a representative LSD and the most common cause of pediatric dementia, patients develop blindness, seizures, and dementia. Several CLN3 disease mouse models have been developed. While all recapitulate the storage accumulation seen in patients, behavioral phenotypes are subtle and variable. To overcome this limitation, we identified robust, reproducible phenotypes on network-level electrophysiology studies in two CLN3 models, a knockout and a human mutation model. Unlike histopathology, physiologic measures directly reflect function and, therefore, may be a better readout for therapy development. Our work suggests CLN3 disease, traditionally considered a degenerative disorder, disrupts early neurodevelopment, especially in the hippocampus, a vulnerable region in CLN3 disease. On in vitro voltage sensitive dye imaging (VSDI) and in vivo electroencephalogram (EEG) recordings, Cln3-/- mice have decreased excitability of the hippocampal dentate gyrus (DG), faster EEG background activity, and loss of hippocampal sharp wave ripples, oscillations that encode new memories. Also, DG neurogenesis is upregulated, perhaps as a compensatory mechanism, early (2mo) but not later (6mo) in disease. Similar network changes arise in other models of neurodegeneration including Alzheimer’s disease (AD). Deep brain stimulation of the entorhinal cortex has been shown to improve outcomes in mouse models of AD. Previously, we found that very early Cln3 gene replacement at p0 corrects network dynamics in a Cln3 knockout mouse. Our central hypothesis is that abnormal neuronal circuit development will limit the window of time, i.e. ‘therapeutic window’, when gene replacement will improve network physiology in CLN3 disease. Furthermore, we predict that altering activity in a key circuit could modify the therapeutic window and efficacy of gene therapy. Our Specific Aims are to: 1) define abnormal dentate gyrus development in CLN3 disease mice, 2) determine the therapeutic window for correction of hippocampal circuit dynamics by gene replacement, and 3) test if modifying entorhinal cortex activity alters circuit defects and response to gene therapy. In this way we will use CLN3 Disease as a representative LSD, to explore the relationship between network activity and response to gene therapy. Our long-term goal is to develop network-directed therapies that, when combined with gene replacement, improve outcomes in LSDs.