Notch signaling in cardiovascular disease and calcification

Abstract

Recent increase in human lifespan has shifted the spectrum of aging-related disorders to an unprecedented upsurge in cardiovascular diseases, especially calcific aortic valve stenosis, which has an 80% risk of progression to heart failure and death. A current therapeutic option for calcified valves is surgical replacement, which provides only temporary relief. Recent progress in cardiovascular research has suggested that arterial and valve calcification are the result of an active process of osteogenic differentiation, induced by a pro-atherogenic inflammatory response. At molecular level, the calcification process is regulated by a network of signaling pathways, including Notch, Wnt and TGFbeta/BMP pathways, which control the master regulator of osteogenesis Cbfa1/Runx2. Genetic and in vitro studies have implicated Notch signaling in the regulation of macrophage activation and cardiovascular calcification. Individuals with inactivating Notch1 mutations have a high rate of cardiovascular disorders, including valve stenosis and calcification. This article reviews recent progress in the mechanism of cardiovascular calcification and discusses potential molecular mechanisms involved, focusing on Notch receptors. We propose a calcification model where extreme increases in vascular wall cell density due to inflammation-induced cell proliferation can trigger an osteogenic differentiation program mediated by Notch receptors.

Keywords: Calcification; Notch signaling; atherosclerosis.; cardiac valve; inflammation; mesenchymal stem cells.

Fig. (1)

Diagram of inflammation-induced osteogenic differentiation. Changes in cellular phenotypes (open arrows) are shown in an integrated network with overlapping regulatory arrays of signal transduction (solid arrows), which form signaling hubs specific for each cell type. Major signaling pathways (Wnt, TGF-beta, BMP, Runx2-ALP) that interact with Notch are shown in boxes and major inflammatory molecules (NO, TNF-alpha, NFkappaB, HIF1) are in bold. Inflammation, initiated by fat accumulation, elastin degradation or pathogens, induces the activation and proliferation of macrophages. Activated macrophages interact with, and induce the proliferation of neighboring vascular smooth muscle (VSMCs) cells or resident stem cells (MSCs) through secreted inflammatory molecules as well as direct contact via Notch receptors and ligands. The balance between Notch and Wnt pathways, with regulatory contributions from inflammatory molecules, TGF-beta and bone morphogenetic proteins (BMPs), controls the proliferation/ differentiation of VSMCs or MSCs. High cell density generated by initial proliferative phase can tip the balance towards activation of Runx2/ALP and osteogenic differentiation.

Fig. (2)

Osteogenesis associates with inflammation in early-stage atherosclerosis. A, Sequential intravital fluorescence microscopy of ApoE KO mouse carotid atherosclerotic plaques. ApoE KO mice fed a high-cholesterol diet were imaged in vivo at 20 weeks old, using multichannel laser scanning fluorescence microscopy for simultaneous visualization of osteogenesis (750 nm, red) and inflammation (680 nm, green). Mice were then randomized to continue with high-cholesterol diet with the addition or absence of statin for an additional 10 weeks. B, Quantification of inflammation and osteogenesis shown in Fig. 2A, demonstrating that statin treatment reduced by more than 50% areas of both inflammation and calcification at 30 weeks.

Fig. (3)

Simplified diagram of Notch signaling. Ligand binding to Notch receptors induces sequential receptor cleavage on the extracellular and intracellular sides of the membrane by TACE/ADAM and γ-secretase/presenilin proteases. The Notch intracellular domain (NICD) migrates to the nucleus and acts as a transcription regulator for cardiovascular/osteogenic or neuronal differentiation pathways. Classical Notch ligands induce signaling pathways that play a role in cardiovascular development and disease, including calcification, through CSL, HES/Hey and Runx2-regulated transcription. Notch glycosylation by Fringe favors binding of Delta-like ligands and inhibits binding of Jagged ligands. Atypical Notch ligands are involved in neuronal development and disease through Deltex-regulated transcription. The two pathways are mutually exclusive through reciprocal negative feedbacks. Proteins with a known role in cardiovascular development are shown underlined. Proteins with known roles in cardiovascular calcification are shown italics.