Increased cell and matrix accumulation during atherogenesis in mice with vessel wall-specific deletion of discoidin domain receptor 1.

RATIONALE: Discoidin domain receptor (DDR)1 is a collagen receptor expressed on both smooth muscle cells (SMCs) and macrophages, where it plays important roles regulating cell and matrix accumulation during atherogenesis. Systemic deletion of DDR1 resulted in attenuated plaque growth but accelerated matrix accumulation in LDLR-deficient mice. Deletion of DDR1 solely on bone marrow-derived cells resulted in decreased macrophage accumulation and plaque growth but no change in matrix accumulation. OBJECTIVE: These findings led us to hypothesize that accelerated matrix accumulation was attributable to the increased synthetic ability of Ddr1(-/-) resident vascular wall SMCs. METHODS AND RESULTS: We used bone marrow transplantation to generate chimeric mice and investigate the role of SMC DDR1 during atherogenesis. Mice with deficiency of DDR1 in vessel wall-derived cells (Ddr1(+/+-->-/-)) or control mice (Ddr1(+/+-->+/+)) were fed an atherogenic diet for 12 weeks. We observed a 3.8-fold increase in the size of aortic sinus plaques in Ddr1(+/+-->-/-) compared to Ddr1(+/+-->+/+) mice. This was attributed to pronounced accumulation of collagen, elastin, proteoglycans, and fibronectin and resulted in a thickened fibrous cap. The enhanced matrix accumulation decreased the proportion of plaque area occupied by cells but was associated with a shift in the cellular composition of the lesions toward increased numbers of vessel wall-derived SMCs compared to bone marrow-derived macrophages. In vitro studies confirmed that Ddr1(-/-) SMCs expressed more matrix, proliferated more, and migrated farther than Ddr1(+/+) SMCs. CONCLUSIONS: DDR1 expression on resident vessel wall SMCs limits proliferation, migration and matrix accumulation during atherogenesis.

Discoidin domain receptor-1 deficiency attenuates atherosclerotic calcification and smooth muscle cell-mediated mineralization.

Intimal calcification is a feature of advanced atherosclerotic disease that predicts a two- to eightfold increase in the risk of coronary events. Type I collagen promotes vascular smooth muscle cell-mediated calcification, although the mechanism by which this occurs is unknown. The discoidin domain receptor 1 (DDR1) is a collagen receptor that is emerging as a critical mediator of atherosclerosis. To determine whether DDR1 is involved in intimal calcification, we fed male Ddr1(-/-);Ldlr(-/-) and Ddr1(+/+);Ldlr(-/-) mice an atherogenic diet for 6, 12, or 24 weeks. DDR1 deficiency significantly reduced the calcium content of the aortic arch, and microcomputed tomography demonstrated a significant decrease in hydroxyapatite deposition after 24 weeks of atherogenic diet. Reduced calcification was correlated with decreases in macrophage accumulation and tumor necrosis factor alpha staining, suggesting that the reduction in calcification was in part due to decreased inflammation. The chondrogenic markers type II collagen, type X collagen, and Sox-9 were expressed within the mineralized foci. An in vitro assay performed with vascular smooth muscle cells revealed that DDR1 was required for cell-mediated calcification of the matrix, and Ddr1(+/+) smooth muscle cells expressed more alkaline phosphatase activity, whereas Ddr1(-/-) smooth muscle cells expressed elevated levels of mRNA for nucleotide pyrophosphatase phosphodiesterase 1, an inhibitor of tissue mineralization. Taken together, our results demonstrate that DDR1 mediates an important mechanism for atherosclerotic calcification.

Discoidin domain receptor 1 (ddr1) deletion decreases atherosclerosis by accelerating matrix accumulation and reducing inflammation in low-density lipoprotein receptor-deficient mice.

Collagens are abundant within the atherosclerotic plaque, where they contribute to lesion volume and mechanical stability and influence cell signaling. The discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase that binds to collagen, is expressed in blood vessels, but evidence for a functional role during atherogenesis is incomplete. In the present study, we generated Ddr1(+/+);Ldlr(-/-) and Ddr1(-/-);Ldlr(-/-) mice and fed them an atherogenic diet for 12 or 24 weeks. Targeted deletion of Ddr1 resulted in a 50% to 60% reduction in atherosclerotic lesion area in the descending aorta at both 12 and 24 weeks. Ddr1(-/-);Ldlr(-/-) plaques exhibited accelerated deposition of fibrillar collagen and elastin at 12 weeks compared with Ddr1(+/+);Ldlr(-/-) plaques. Expression analysis of laser microdissected lesions in vivo, and of Ddr1(-/-) smooth muscle cells in vitro, revealed increased mRNA levels for procollagen alpha1(I) and alpha1(III) and tropoelastin, suggesting an enhancement of matrix synthesis in the absence of DDR1. Furthermore, whereas plaque smooth muscle cell content was unchanged, Ddr1(-/-);Ldlr(-/-) plaques had a 49% decrease in macrophage content at 12 weeks, with a concomitant reduction of in situ gelatinolytic activity. Moreover, mRNA expression of both monocyte chemoattractant protein-1 and vascular cell adhesion molecule-1 was reduced in vivo, and Ddr1(-/-);Ldlr(-/-) macrophages demonstrated impaired matrix metalloproteinase expression in vitro. These data suggest novel roles for DDR1 in macrophage recruitment and invasion during atherogenesis. In conclusion, our data support a role for DDR1 in the regulation of both inflammation and fibrosis early in plaque development. Deletion of DDR1 attenuated atherogenesis and resulted in the formation of matrix-rich plaques.

Collagens in the progression and complications of atherosclerosis.

Collagens constitute a major portion of the extracellular matrix in the atherosclerotic plaque, where they contribute to the strength and integrity of the fibrous cap, and also modulate cellular responses via specific receptors and signaling pathways. This review focuses on the diverse roles that collagens play in atherosclerosis; regulating the infiltration and differentiation of smooth muscle cells and macrophages; controlling matrix remodeling through feedback signaling to proteinases; and influencing the development of atherosclerotic complications such as plaque rupture, aneurysm formation and calcification. Expanding our understanding of the pathways involved in cell-matrix interactions will provide new therapeutic targets and strategies for the diagnosis and treatment of atherosclerosis.