Accumulation of Notch3 extracellular domain and not aberrant NOTCH3 signaling is the key driver of CADASIL arterial pathology
Loss of arterial smooth muscle cells (SMCs) and abnormal accumulation of the extracellular domain of the NOTCH3 (Notch3ECD) receptor are the two core features of CADASIL, the most common genetic cerebral small vessel disease. Although CADASIL is known to be caused by highly stereotyped dominant mutations in NOTCH3, the relationship between NOTCH3 receptor activity, Notch3ECD  accumulation and arterial SMC loss has remained elusive, hampering the development of disease-modifying therapies. Using novel histopathological and multiscale imaging modalities, we quantified previously undetectable CADASIL-driven focal arterial SMC loss in the central nervous system of mice expressing the archetypal Arg169Cys CADASIL mutation. Notably, we found more severe arterial pathology and greater Notch3ECD accumulation in transgenic model mice overexpressing the mutation on a homozygous wild-type Notch3 background (TgNotch3R169C) than in knock-in model Notch3R170C/R170C mice expressing this mutation without a wild-type Notch3 copy. We further showed that wild-type Notch3ECD co-aggregated with mutant Notch3ECD and that elimination of one copy of wild-type Notch3 in TgNotch3R169C mice was sufficient to attenuate Notch3ECD accumulation and arterial pathology. Importantly, RNA sequencing revealed no substantial change in the expression of Notch3-regulated genes in TgNotch3R169C brain arteries. Collectively, our results provide compelling evidence that Notch3ECD accumulation, involving mutant and wild-type NOTCH3, and not aberrant NOTCH3 signaling, is the key driver of arterial SMC loss in CADASIL, thus identifying NOTCH3-lowering approaches as candidate therapeutic strategies. Overall design: To disentangle whether a loss or a gain of NOTCH3 activity or a neomorphic effect are involved in CADASIL-associated arterial SMC loss, we profiled the transcriptome of brain arteries dissected from TgNotch3R169C aged 12 months, Notch3 deficient (Notch3-/-) aged 3 months and their WT respective age-matched littermates using bulk RNA-sequencing. We then performed comparative gene expression profiling of RNAseq data.