Abstract
Rationale Rupture of fibrous caps in atherosclerotic lesions is the underlying cause of the majority of thrombi leading to heart attack and a frequent cause of stroke. Caps are produced by SMCs that are recruited to the subendothelial space. We hypothesized that the recruitment mechanisms are shared with embryonic artery development, which rely prominently on Notch signaling for forming the subendothelial layers of medial SMCs.
Objective To analyze the causal roles of the Notch signaling pathway in SMCs during atherogenesis and cap formation.
Methods and Results Notch elements involved in arterial media development were identified in regions of fibrous cap formation in murine plaques. To analyze the functional effect of Notch signaling in cap formation, we induced atherosclerosis in mice in which Notch pathway was blocked specifically in SMCs by conditional knockout of the required transcription factor RBPJ. No major effects on atherosclerotic plaque size or necrosis were observed, but the presence of cap SMCs was significantly reduced. Lineage tracing revealed that the accumulation of SMC-derived plaque cells in the cap region was unaltered but that Notchdefective cells failed to re-acquire SMC phenotype in the cap. To analyze whether the accumulation of SMC-derived cells to atherogenesis requires down-regulation of Notch signaling, we studied atherosclerosis in mice with constitutive Notch signaling in SMCs achieved by conditional expression of a Notch intracellular domain. Forced Notch signaling inhibited the ability of medial SMCs to contribute to plaque cells, including both cap SMCs and osteochondrogenic cells, and it significantly reduced the development of atherosclerosis.
Conclusions Sequential loss and gain of Notch signaling is necessary for building the contractile cap SMC population. The shared mechanisms with embryonic arterial media assembly suggests that the fibrous cap may form as a neo-media to restore the connection between endothelium and SMCs in the developing plaque.
Competing Interest Statement
The authors have declared no competing interest.