ABSTRACT Cerebral small vessel disease (SVD) is an important cause of dementia and contributes to neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is the most common inherited cause of SVD. Clinical features of CADASIL include dementia, recurrent stroke, and psychiatric disorders. Understanding the pathology of CADASIL may lead to strategies to treat SVD. CADASIL is caused by cysteine-altering mutations in NOTCH3, suggesting that redox alteration of this protein could contribute to SVD. Altered forms of NOTCH3 may lead to massive protein accumulation, cell separation, and smooth muscle cell death seen in CADASIL. Specific cell loss, protein conformational alterations, and protein accumulation seen in CADASIL suggest similarities to degenerative disorders such as Alzheimer's and Parkinson's diseases. In preliminary studies, we identify a novel post- translationally generated fragment of NOTCH3, called NTF (NOTCH3 N-terminal fragment) from human CADASIL tissues. We show that NTF is capable of trans- reducing NOTCH3 into a pathological form of NOTCH3 with multiple reduced cysteines (mrc-N3). Moreover, NTF causes both acute and chronic changes in the brains of mice which are not normally susceptible to SVD. Enzymatic liberation of NTF occurs in cells, and we identify a potential protease that is increased in CADASIL tissues. In Aim 1, we will define the features of NTF that promote NOTCH3 protein reduction to mrc-N3. In Aim 2, we will we explore the extent of protein reduction mediated by NTF. And in Aim 3, we will determine whether a brain protease that is enriched in CADASIL participates in NTF generation. These experiments explore a novel, self-perpetuating molecular mechanism by which pathological proteins are produced in the human brain. Information generated by this project could improve current translational models of SVD and enable progress in the discovery of therapeutic targets.