Traditional imaging modalities such as computed tomography, although perfectly adept at identifying and quantifying advanced calcification, cannot detect the early stages of this disorder and offer limited insight into the mechanisms of mineral dysregulation. This review presents optical molecular imaging as a promising tool that simultaneously detects pathobiological processes associated with inflammation and early stages of calcification in vivo at the (sub)cellular levels. Research into treatment of cardiovascular calcification is lacking, as shown by clinical trials that have failed to demonstrate the reduction of calcific aortic stenosis. Hence the need to elucidate the pathways that contribute to cardiovascular calcification and to develop new therapeutic strategies to prevent or reverse calcification has driven investigations into the use of molecular imaging. This review discusses studies that have used molecular imaging methods to advance knowledge of cardiovascular calcification, focusing in particular on the inflammation-dependent mechanisms of arterial and aortic valve calcification.

Rationale We previously showed that early calcification of atherosclerotic plaques associates with macrophage accumulation. Chronic renal disease (CRD) and mineral imbalance accelerates calcification and the subsequent release of matrix vesicles (MVs) — precursors of microcalcification. Objective We tested the hypothesis that macrophage-derived MVs contribute directly to microcalcification. Methods and Results Macrophages associated with regions of calcified vesicular structures in human carotid plaques (n=136 patients). In vitro, macrophages released MVs with high calcification and aggregation potential. MVs expressed exosomal markers (CD9 and TSG101), and contained S100A9 and annexin V (Anx5). Silencing S100A9 in vitro and genetic deficiency in S100A9−/− mice reduced MV calcification, while stimulation with S100A9 increased calcification potential. Externalization of phosphatidylserine (PS) after Ca/P stimulation and interaction of S100A9 and Anx5, indicated that a PS-Anx5-S100A9 membrane complex facilitates hydroxyapatite nucleation within the macrophage-derived MV membrane. Conclusions Our results support the novel concept that macrophages release calcifying MVs enriched in S100A9 and Anx5, which contribute to accelerated microcalcification in CRD.

Cardiovascular calcification is an independent risk factor for cardiovascular morbidity and mortality. This disease of dysregulated metabolism is no longer viewed as a passive degenerative disease, but instead as an active process triggered by pro-inflammatory cues. Furthermore, a positive feedback loop of calcification and inflammation is hypothesized to drive disease progression in arterial calcification. Both calcific aortic valve disease and atherosclerotic arterial calcification may possess similar underlying mechanisms. Early histopathological studies first highlighted the contribution of inflammation to cardiovascular calcification by demonstrating the accumulation of macrophages and T lymphocytes in 'early' lesions within the aortic valves and arteries. A series of in vitro work followed, which gave a mechanistic insight into the stimulation of smooth muscle cells to undergo osteogenic differentiation and mineralization. The emergence of novel technology, in the form of animal models and more recently molecular imaging, has enabled accelerated progression of this field, by providing strong evidence regarding the concept of this disorder as an inflammatory disease. Although there are still gaps in our knowledge of the mechanisms behind this disorder, this review discusses the various studies that have helped form the concept of the inflammation-dependent cardiovascular calcification paradigm. (Circ J 2011; 75: 1305 - 1313

Extracellular vesicles are membrane micro/nanovesicles secreted by many cell types into the circulation and the extracellular milieu in physiological and pathological conditions. Evidence suggests that extracellular vesicles, known as matrix vesicles, play a role in the mineralization of skeletal tissue, but emerging ultrastructural and in vitro studies have demonstrated their contribution to cardiovascular calcification as well. Cells involved in the progression of cardiovascular calcification release active vesicles capable of nucleating hydroxyapatite on their membranes. This review discusses the role of extracellular vesicles in cardiovascular calcification, and elaborates on this additional mechanism of calcification as an alternative pathway to the currently accepted mechanism of biomineralization via osteogenic differentiation.