PROJECT ABSTRACT The GGGGCC (G4C2) hexanucleotide repeat expansion (HRE) in the first intron of the gene C9orf72, is the most common genetic abnormality associated with frontotemporal dementia (FTD) and amyotrophic lateral sclerosis (ALS). The disease pathogenesis ultimately leads to the concurrent degeneration of cortical forebrain and spinal motor neurons, and result in the clinical deficits of motor function and dementia. While the C9orf72- FTD/ALS disease pathogenesis has been well characterized in spinal motor neurons and a contribution of the observed neurodegeneration has been attributed to spinal cord astrocytes, there is little known about the pathobiology in cortical astrocytes and their role in cortical neurodegeneration, which is proposed to contribute to the dementia symptoms in this patient population. In the parent grant of this supplement, we hypothesized that cortical astrocytes play an integral role in the non-cell autonomous disease pathology contributing to the degeneration of cortical neurons in C9orf72-FTD/ALS. To test this hypothesis, we proposed cellular and molecular analyses of postmortem forebrain autopsy tissues and patient-derived iPSC cortical neurons and cortical astrocytes co-culture systems. With this supplement, we expand on these studies and propose to examine the contribution of astrocyte-neuron contact-dependent mechanisms (Aim 1) and astrocyte-secreted factors (Aim 2) in cortical neurodegeneration. These contributions will be tested using iPSC cortical astrocyte- cortical neuron co-culture models. The graduate student assigned to this project, Ms. Lynette Bustos, will focus on known astrocyte proteins (e.g., neuroligins and ephrins) that make direct contact with neuronal synaptic proteins, as well as astrocyte secreted proteins implicated in synapse structure and function (e.g. Hevin, SPARC, thrombospondins, glypicans). Lynette will thoroughly examine the role of these proteins in the degeneration of cortical neurons. Together with the studies performed under the parent grant, these analyses will for the first time elucidate the contributing role of cortical astrocytes in the neurodegeneration of cortical neurons in C9orf72- FTD/ALS, addressing the disease mechanisms of dementia in this spectrum disorder. Additionally, this work will provide novel opportunities for drug target identification with the hope of identifying novel therapeutics for the affected patient populations.