PROJECT SUMMARY The objective of this application is to establish a novel framework to support investigations into the role of astrocytes in neurological disease, including neurodegeneration, as well as vascular contributions to cognitive impairment and dementia (VCID). The application is motivated by observations linking astrocyte dysfunction to cerebrovascular disease. We propose to investigate the role of astrocytes in cerebrovascular malformations by systematically dissecting the functional relevance of CCM3 (Cerebral cavernous malformation 3) in model organisms and primary cultures. Loss- of-function mutations in CCM3 cause the most aggressive form of familial Cerebral Cavernous Malformations (CCM), a common cerebrovascular disease of seizures and hemorrhagic stroke characterized by the presence of vascular lesions (known as CCMs, cavernous angiomas, or cavernomas) in the central nervous system. Our ongoing research in mouse models suggested that CCM3 has unique and specific functions in astrocytes, in addition to its established role in vascular endothelial cells. CCM3 deficiency in radial glia progenitors and astrocytes has cell-intrinsic effects, leading to astrocyte activation and dysfunction. In addition, it has cell non-autonomous effects, and results in generalized cerebral vascular pathology and formation of CCM-like lesions in mice. The proposed studies take advantage of animal models we generated, as well as primary cultures of astrocytes, aiming to investigate the consequences of CCM3 deficiency in astrocytes at the structural and molecular level, with attention to astrocyte reactivity and cholesterol production, both of which have been linked to other neurological and neurodegenerative disorders. By elucidating cell intrinsic functions of CCM3 in astrocytes and defining the molecular cascade that mediates its actions, we aim to understand how astrocyte dysfunction contributes to pathology. Our studies carry basic biological value and, by extending mechanistic understanding of CCM3 action, will allow us to gain what may be therapeutically important insights into CCM disease. Moreover, our findings have the potential to establish a genetically tractable system in which to investigate reactive astrocyte dysfunction not only in CCM, but in other neurological diseases, including VCID and neurodegeneration.