PROJECT SUMMARY Aberrant glial activation is a prominent feature in neurodegenerative diseases. But, what triggers glial activation in the aging brain and how it contributes to neuronal degeneration remains unclear. The scientific premise of this proposal is based on previous studies that dominant mutations in human Progranulin gene (GRN [gene], PGRN [protein]) cause a drastic reduction in PGRN levels in CSF and brain tissues in patients with frontotemporal lobar degeneration (FTLD), leading to profound gliosis, aggregation of RNA binding protein TDP-43, and neurodegeneration. In support of this idea, our recent studies show that Grn knockout (Grn-/-) mice is a valid model that captures several key disease features in FTLD caused by GRN mutations (FTLD- GRN), including microglial activation, microglia-mediated synaptic pruning and dysfunction in the thalamocortical circuit. Our ongoing work further revealed that Grn-/- mice and FTLD-GRN patients also shows a robust astroglial activation that positively correlates with microglial activation. Similar to Grn-/- microglia, Grn-/- astrocytes exhibit an age-dependent up-regulation of innate immunity genes, including complements C3 and C4b, which together with C1qa from Grn-/- microglia, activate both classical and alternative complement pathways to promote neurodegeneration. Taken together, these results support the hypothesis that PGRN deficiency is a feasible disease model to uncover the intricate neuroimmune interactions and how perturbation to these interactions leads to neuronal degeneration. To test this hypothesis, we propose a comprehensive single cell transcriptomic approach to survey the dynamic changes of glial and neuronal cell types in the thalamocortical circuit that is most severely impacted by PGRN deficiency. This approach will provide critical insights into the intrinsic mechanism of glial activation, neuronal degeneration and neural circuit dysfunction in Grn-/- mice and in FTLD-GRN patients. This innovative strategy involves high throughput profiling of transcriptomic and physiological properties of glia and neurons using droplet-based capture technology, microscopy and dynamic imaging of cell intrinsic physiological responses. These results will provide an unprecedented resolution to directly test the hypothesis that disruptions to the dynamic neuroimmune interactions between microglia, astrocytes and neurons in the thalamocortical circuit lead to neurodegeneration in FTLD caused by PGRN deficiency.