Osteogenesis associates with inflammation in early-stage atherosclerosis evaluated by molecular imaging in vivo

Calcific aortic valve stenosis: methods, models, and mechanisms

Calcific aortic valve stenosis (CAVS) is a major health problem facing aging societies. The identification of osteoblast-like and osteoclast-like cells in human tissue has led to a major paradigm shift in the field. CAVS was thought to be a passive, degenerative process, whereas now the progression of calcification in CAVS is considered to be actively regulated. Mechanistic studies examining the contributions of true ectopic osteogenesis, nonosseous calcification, and ectopic osteoblast-like cells (that appear to function differently from skeletal osteoblasts) to valvular dysfunction have been facilitated by the development of mouse models of CAVS. Recent studies also suggest that valvular fibrosis, as well as calcification, may play an important role in restricting cusp movement, and CAVS may be more appropriately viewed as a fibrocalcific disease. High-resolution echocardiography and magnetic resonance imaging have emerged as useful tools for testing the efficacy of pharmacological and genetic interventions in vivo. Key studies in humans and animals are reviewed that have shaped current paradigms in the field of CAVS, and suggest promising future areas for research.

Molecular imaging insights into early inflammatory stages of arterial and aortic valve calcification

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.

Microcalcifications in early intimal lesions of atherosclerotic human coronary arteries

Although calcium (Ca) precipitation may play a pathogenic role in atherosclerosis, information on temporal patterns of microcalcifications in human coronary arteries, their relation to expression of calcification-regulating proteins, and colocalization with iron (Fe) and zinc (Zn) is scarce. Human coronary arteries were analyzed post mortem with a proton microprobe for element concentrations and stained (immuno)histochemically for morphological and calcification-regulating proteins. Microcalcifications were occasionally observed in preatheroma type I atherosclerotic intimal lesions. Their abundance increased in type II, III, and IV lesions. Moreover, their appearance preceded increased expression of calcification-regulating proteins, such as osteocalcin and bone morphogenetic protein-2. In contrast, their presence coincided with increased expression of uncarboxylated matrix Gla protein (MGP), whereas the content of carboxylated MGP was increased in type III and IV lesions, indicating delayed posttranslational conversion of biologically inactive into active MGP. Ca/phosphorus ratios of the microcalcifications varied from 1.6 to 3.0, including amorphous Ca phosphates. Approximately 75% of microcalcifications colocalized with the accumulation of Fe and Zn. We conclude that Ca microprecipitation occurs in the early stages of atherosclerosis, inferring a pathogenic role in the sequel of events, resulting in overt atherosclerotic lesions. Microcalcifications may be caused by local events triggering the precipitation of Ca rather than by increased expression of calcification-regulating proteins. The high degree of colocalization with Fe and Zn suggests a mutual relationship between these trace elements and early deposition of Ca salts.

Notch signaling pathway enhances bone morphogenetic protein 2 (BMP2) responsiveness of Msx2 gene to induce osteogenic differentiation and mineralization of vascular smooth muscle cells

Vascular calcification is regulated in a process similar to bone formation. BMP2 (bone morphogenetic protein 2) is essential for osteoblastic differentiation of mesenchymal progenitor cells and thus has been implicated in the development of vascular calcification. Here we examined whether Notch signaling interacts with BMP2 signaling to regulate osteogenic differentiation and mineralization of vascular smooth muscle cells (SMCs). BMP2 alone scarcely induced the expression of alkaline phosphatase (ALP), an ectoenzyme crucially required for active biomineralization, in human aortic SMCs (HASMCs), despite its strong induction in osteoblast precursor MC3T3-E1 cells. Notably, overexpression of the Notch1 intracellular domain (N1-ICD) markedly enhanced BMP2-mediated induction of ALP activity and mineralization of HASMCs. In HASMCs, expression of Msx2 gene, a well documented BMP2 target gene in osteoblasts, was barely induced by BMP2 alone, and N1-ICD clearly enhanced the BMP2-driven Msx2 gene expression. Deletion and site-directed mutation analysis of Msx2 gene promoter revealed that the RBPJk-binding site was necessary for BMP2 responsiveness. Using the RBPJk-deficient cells and siRNA for RBPJk, we showed that RBPJk was required for BMP2 induction of Msx2 gene expression and ALP activity. Moreover, we showed that Smad1, a transcription factor downstream of BMP2 signaling, interacted with N1-ICD to form a complex within the Msx2 promoter. Immunohistochemistry of human calcifying atherosclerotic plaques revealed colocalized expression of Notch1, BMP2, and Msx2. These results indicate that the Notch intracellular domain·RBPJk complex enhances the BMP2-induced Msx2 gene expression by cooperating with Smad1 and suggest that Notch signaling makes vascular SMC responsive to BMP2 and promotes vascular calcification.

Angiotensin II and the vascular phenotype in hypertension

Hypertension is associated with vascular changes characterised by remodelling, endothelial dysfunction and hyperreactivity. Cellular processes underlying these perturbations include altered vascular smooth muscle cell growth and apoptosis, fibrosis, hypercontractility and calcification. Inflammation, associated with macrophage infiltration and increased expression of redox-sensitive pro-inflammatory genes, also contributes to vascular remodelling. Many of these features occur with ageing, and the vascular phenotype in hypertension is considered a phenomenon of ‘premature vascular ageing’. Among the many factors involved in the hypertensive vascular phenotype, angiotensin II (Ang II) is especially important. Ang II, previously thought to be the sole effector of the renin–angiotensin system (RAS), is converted to smaller peptides [Ang III, Ang IV, Ang-(1-7)] that are biologically active in the vascular system. Another new component of the RAS is the (pro)renin receptor, which signals through Ang-II-independent mechanisms and might influence vascular function. Ang II mediates effects through complex signalling pathways on binding to its G-protein-coupled receptors (GPCRs) AT1R and AT2R. These receptors are regulated by the GPCR-interacting proteins ATRAP, ARAP1 and ATIP. AT1R activation induces effects through the phospholipase C pathway, mitogen-activated protein kinases, tyrosine kinases/phosphatases, RhoA/Rhokinase and NAD(P)H-oxidase-derived reactive oxygen species. Here we focus on recent developments and new research trends related to Ang II and the RAS and involvement in the hypertensive vascular phenotype.

Cell biological and physicochemical aspects of arterial calcification

Processes similar to endochondral or intramembranous bone formation occur in the vascular wall. Bone and cartilage tissue as well as osteoblast- and chondrocyte-like cells are present in calcified arteries. As in bone formation, apoptosis and matrix vesicles play an important role in the initiation of vascular calcification. Recent evidence indicates that nanocrystals initially formed in the vessel wall may actively be involved in the progression of the calcification process. This review focuses on the cellular and structural similarities between bone formation and vascular calcification and discusses the initial events in this pathological mineralization process.

Widespread increase in myeloid calcifying cells contributes to ectopic vascular calcification in type 2 diabetes

RATIONALE

Acquisition of a procalcific phenotype by resident or circulating cells is important for calcification of atherosclerotic plaques, which is common in diabetes.

OBJECTIVE

We aim to identify and characterize circulating calcifying cells, and to delineate a pathophysiological role for these cells in type 2 diabetes.

METHODS AND RESULTS

We demonstrate for the first time that a distinct subpopulation of circulating cells expressing osteocalcin and bone alkaline phosphatase (OC(+)BAP(+)) has procalcific activity in vitro and in vivo. The study of naïve patients with chronic myeloid leukemia indicated that OC(+)BAP(+) cells have a myeloid origin. Myeloid calcifying OC(+)BAP(+) cells (MCCs) could be differentiated from peripheral blood mononuclear cells, and generation of MCCs was closely associated with expression of the osteogenic transcription factor Runx2. In gender-mismatched bone marrow-transplanted humans, circulating MCCs had a much longer half-life compared with OC(-)BAP(-) cells, suggesting they belong to a stable cell repertoire. The percentage of MCCs was higher in peripheral blood and bone marrow of type 2 diabetic patients compared with controls but was lowered toward normal levels by optimization of glycemic control. Furthermore, diabetic carotid endoarterectomy specimens showed higher degree of calcification and amounts of cells expressing OC and BAP in the α-smooth muscle actin-negative areas surrounding calcified nodules, where CD68(+) macrophages colocalize. High glucose increased calcification by MCCs in vitro, and hypoxia may regulate MCC generation in vitro and in vivo.

CONCLUSIONS

These data identify a novel type of blood-derived procalcific cells potentially involved in atherosclerotic calcification of diabetic patients.

A selective matrix metalloproteinase-12 inhibitor retards atherosclerotic plaque development in apolipoprotein E-knockout mice

OBJECTIVE

Matrix metalloproteinase (MMP)-12 has been implicated in plaque progression and instability and is also amenable to selective inhibition. In this study, we investigated the influence of a greater than 10-fold selective synthetic MMP-12 inhibitor on plaque progression in the apolipoprotein E knockout mouse model of atherosclerosis.

METHODS AND RESULTS

A phosphinic peptide (RXP470.1) that is a potent, selective murine MMP-12 inhibitor significantly reduced atherosclerotic plaque cross-sectional area by approximately 50% at 4 different vascular sites in male and female apolipoprotein E knockout mice fed a Western diet. Furthermore, RXP470.1 treatment resulted in less complex plaques with increased smooth muscle cell:macrophage ratio, less macrophage apoptosis, increased cap thickness, smaller necrotic cores, and decreased incidence of calcification. Additional in vitro and in vivo findings indicate that attenuated monocyte/macrophage invasion and reduced macrophage apoptosis probably underlie the beneficial effects observed on atherosclerotic plaque progression with MMP-12 inhibitor treatment.

CONCLUSIONS

Our data demonstrate that a selective MMP-12 inhibitor retards atherosclerosis development and results in a more fibrous plaque phenotype in mice. Our study provides proof of principle to motivate translational work on MMP-12 inhibitor therapy in humans.

Fibroblast activation protein is induced by inflammation and degrades type I collagen in thin-cap fibroatheromata

Aims

Collagen degradation in atherosclerotic plaques with thin fibrous caps renders them more prone to rupture. Fibroblast activation protein (FAP) plays a role in arthritis and tumour formation through its collagenase activity. However, the significance of FAP in thin-cap human fibroatheromata remains unknown.

Methods and results

We detected enhanced FAP expression in type IV–V human aortic atheromata (n = 12), compared with type II–III lesions (n = 9; P < 0.01) and healthy aortae (n = 8; P < 0.01) by immunostaining and western blot analyses. Fibroblast activation protein was also increased in thin-cap (<65 µm) vs. thick-cap (≥65 µm) human coronary fibroatheromata (n = 12; P < 0.01). Fibroblast activation protein was expressed by human aortic smooth muscle cells (HASMC) as shown by colocalization on immunofluorescent aortic plaque stainings (n = 10; P < 0.01) and by flow cytometry in cell culture. Although macrophages did not express FAP, macrophage burden in human aortic plaques correlated with FAP expression (n = 12; R²= 0.763; P < 0.05). Enzyme-linked immunosorbent assays showed a time- and dose-dependent up-regulation of FAP in response to human tumour necrosis factor α (TNFα) in HASMC (n = 6; P < 0.01). Moreover, supernatants from peripheral blood-derived macrophages induced FAP expression in cultured HASMC (n = 6; P < 0.01), an effect abolished by blocking TNFα (n = 6; P < 0.01). Fibroblast activation protein associated with collagen-poor regions in human coronary fibrous caps and digested type I collagen and gelatin in vitro (n = 6; P < 0.01). Zymography revealed that FAP-mediated collagenase activity was neutralized by an antibody directed against the FAP catalytic domain both in HASMC (n = 6; P < 0.01) and in fibrous caps of atherosclerotic plaques (n = 10; P < 0.01).

Conclusion

Fibroblast activation protein expression in HASMC is induced by macrophage-derived TNFα. Fibroblast activation protein associates with thin-cap human coronary fibroatheromata and contributes to type I collagen breakdown in fibrous caps.

Hyperphosphatemia-induced nanocrystals upregulate the expression of bone morphogenetic protein-2 and osteopontin genes in mouse smooth muscle cells in vitro

Vascular calcification, which contributes to cardiovascular disease in patients with uremic hyperphosphatemia, is associated with vascular cell expression of osteogenic genes, including bone morphogenetic protein (BMP)-2 and osteopontin (OPN). High inorganic phosphate levels in vitro stimulate the osteogenic conversion of smooth muscle cells; however, the mechanism governing this is not clear. We found that high-phosphate medium increased the expression of BMP-2 and OPN in mouse smooth muscle cells in culture. However, this effect was lost in the presence of the mineralization inhibitor, pyrophosphate, suggesting a contribution of calcium phosphate crystals. Addition of 1-2 mmol/l phosphate alone to growth medium was sufficient to induce nanosized crystals after 1 day at 37 °C. Isolated crystals were about 160 nm in diameter and had a calcium to phosphate ratio of 1.35, consistent with the hydroxyapatite precursor octacalcium phosphate. Nanocrystal formation increased fourfold in the absence of serum, was blocked by fetuin-A, and was dependent on time and on the concentrations of phosphate and calcium. Purified synthetic hydroxyapatite nanocrystals and isolated high-phosphate-induced nanocrystals, but not nanocrystal-free high-phosphate medium, also induced BMP-2 and OPN. Thus, our results suggest that BMP-2 and OPN are induced by calcium phosphate nanocrystals, rather than soluble phosphate. This mechanism may contribute, in part, to hyperphosphatemia-related vascular cell differentiation and calcification.

Nanocrystals seed calcification in more ways than one

Although much progress has been made in the past five years in understanding the mechanisms leading to accelerated vascular calcification in patients with chronic kidney disease, it remains unclear how an environment high in phosphate can impinge so significantly on the calcification process. The study by Sage et al. highlights an important and novel role for calcium phosphate nanocrystals, produced in a high-phosphate environment, in rapidly driving calcification of vascular smooth muscle cells via enhanced production of bone morphogenetic protein-2.

Rodent models in bone-related research: the relevance of calvarial defects in the assessment of bone regeneration strategies

In vivo research with animal models has been a preferred experimental system in bone-related biomedical research since, by approximation, it allows relevant data gathering regarding physiological and pathological conditions that could be of use to establish more effective clinical interventions. Animal models, and more specifically rodent models, have been extensively used and have contributed greatly to the development and establishment of a wide range of translational approaches aiming to regenerate the bone tissue. In this regard, the calvarial defect model has found great application in basic and applied research, nonetheless the controversial rationalization for the use of critical size defects - defects that are unable to report spontaneous healing - or subcritical size defects in the proposed applications. Accordingly, this work aims to review the advantages and limitations of the use of rodent models in biomedical bone-related research, emphasizing the problematic issues of the use of calvarial critical and subcritical size defects. Additionally, surgical protocols for the establishment of both defects in rat calvarial bone, as well as the description and exemplification of the most frequently used techniques to access the bone tissue repair, are portrayed.

Real-time in vivo detection of biomaterial-induced reactive oxygen species

The non-specific host response to implanted biomaterials is often a key challenge of medical device design. To evaluate biocompatibility, measuring the release of reactive oxygen species (ROS) produced by inflammatory cells in response to biomaterial surfaces is a well-established method. However, the detection of ROS in response to materials implanted in vivo has not yet been demonstrated. Here, we develop a bioluminescence whole animal imaging approach to observe ROS released in response to subcutaneously-implanted materials in live animals. We compared the real-time generation of ROS in response to two representative materials, polystyrene and alginate, over the course of 28 days. High levels of ROS were observed near polystyrene, but not alginate implants, and persisted throughout the course of 28 days. Histological analysis revealed that high levels of ROS correlated not only with the presence of phagocytic cells at early timepoints, but also fibrosis at later timepoints, suggesting that ROS may be involved in both the acute and chronic phase of the foreign body response. These data are the first in vivo demonstration of ROS generation in response to implanted materials, and describe a novel technique to evaluate the host response.

Increased levels of the calcification marker matrix Gla Protein and the inflammatory markers YKL-40 and CRP in patients with type 2 diabetes and ischemic heart disease

OBJECTIVE AND DESIGN

Low grade inflammation is of pathogenic importance in atherosclerosis and in the development of cardiovascular disease (CVD) and type 2 diabetes (T2D). Matrix GLA protein (MGP), an inhibitor of medial calcification of arteries, is increased in patients with atherosclerosis. In the present study levels of markers of calcification (MGP) and inflammation (YKL-40, hsCRP) were evaluated in patients with T2 D and/or ischemic heart disease (IHD).

MATERIALS AND METHODS

The study population consisted of 1) patients with T2D (n = 45); 2) patients with IHD (n = 37); patients with both T2D and IHD (n = 20) and 4) healthy controls (n = 20). Biochemical parameters were measured in venous blood samples.

RESULTS

Levels of MGP, YKL-40 and hsCRP were increased in patients with IHD and/or T2D (p < 0.0001) and patients with T2D and IHD had higher MGP levels (p < 0.001). In multiple linear regression analyses MGP was associated with patient category (r = 0.36, p < 0.001), and HDL-cholesterol levels (r = 0.29, p < 0.001) adjusting for the significant covariates.

CONCLUSIONS

In patients with T2D and/or IHD we found increased levels of plasma MGP indicative of a progressing calcification process. This process is paralleled by increased levels of YKL-40 and hsCRP, which most likely reflect the concomitant low grade inflammatory state in these patients.

Role of extracellular membrane vesicles in the pathogenesis of various diseases, including cancer, renal diseases, atherosclerosis, and arthritis

Extracellular membrane vesicles (MVs) 30-1000 nm in diameter and of varying cellular origins are increasingly recognized for their participation in a range of processes, including the pathogenesis of various diseases, such as: (1) atherosclerosis, (2) thromboembolism, (3) osteoarthritis (OA), (4) chronic renal disease and pulmonary hypertension, (5) tissue invasion and metastasis by cancer cells, (6) gastric ulcers and bacterial infections, and (7) periodontitis. MVs are derived from many different cell types and intracellular mechanisms, and perform different metabolic functions or roles, depending on the cell of origin.The presence of a metabolically active, outer membrane is a distinguishing feature of all MVs, regardless of their cell type of origin and irrespective of terminologies applied to them such as exosomes, microparticles, or matrix vesicles. The MV membrane provides one of the few protected and controlled internal microenvironments outside cells in which specific metabolic objectives of the host cell may be pursued vigorously at a distance from the host cell. MVs are also involved in various forms of normal and abnormal intercellular communication. Evidence is emerging that circulating MVs are good predictors of the severity of several diseases. In addition, recently, the role of MVs in inducing immunity against cancer cells and bacterial infections has become a topic of interest to researchers in the area of therapeutics. The main objective of this review is to list and briefly describe the increasingly well-defined roles of MVs in selected diseases in which they seem to have a significant role in pathogenesis.

Intravital molecular imaging of small-diameter tissue-engineered vascular grafts in mice: a feasibility study

OBJECTIVES

Creating functional small-diameter tissue-engineered blood vessels has not been successful to date. Moreover, the processes underlying the in vivo remodeling of these grafts and the fate of cells seeded onto scaffolds remain unclear. Here we addressed these unmet scientific needs by using intravital molecular imaging to monitor the development of tissue-engineered vascular grafts (TEVG) implanted in mouse carotid artery.

METHODS AND RESULTS

Green fluorescent protein-labeled human bone marrow-derived mesenchymal stem cells and cord blood-derived endothelial progenitor cells were seeded on polyglycolic acid-poly-L-lactic acid scaffolds to construct small-caliber TEVG that were subsequently implanted in the carotid artery position of nude mice (n = 9). Mice were injected with near-infrared agents and imaged using intravital fluorescence microscope at 0, 7, and 35 days to validate in vivo the TEVG remodeling capability (Prosense680; VisEn, Woburn, MA) and patency (Angiosense750; VisEn). Imaging coregistered strong proteolytic activity and blood flow through anastomoses at both 7 and 35 days postimplantation. In addition, image analyses showed green fluorescent protein signal produced from mesenchymal stem cell up to 35 days postimplantation. Comprehensive correlative histopathological analyses corroborated intravital imaging findings.

CONCLUSIONS

Multispectral imaging offers simultaneous characterization of in vivo remodeling enzyme activity, functionality, and cell fate of viable small-caliber TEVG.

S100A12 in vascular smooth muscle accelerates vascular calcification in apolipoprotein E-null mice by activating an osteogenic gene regulatory program

OBJECTIVE

The proinflammatory cytokine S100A12 is associated with coronary atherosclerotic plaque rupture. We previously generated transgenic mice with vascular smooth muscle-targeted expression of human S100A12 and found that these mice developed aortic aneurysmal dilation of the thoracic aorta. In the current study, we tested the hypothesis that S100A12 expressed in vascular smooth muscle in atherosclerosis-prone apolipoprotein E (ApoE)-null mice would accelerate atherosclerosis.

METHODS AND RESULTS

ApoE-null mice with or without the S100A12 transgene were analyzed. We found a 1.4-fold increase in atherosclerotic plaque size and more specifically a large increase in calcified plaque area (45% versus 7% of innominate artery plaques and 18% versus 10% of aortic root plaques) in S100A12/ApoE-null mice compared with wild-type/ApoE-null littermates. Expression of bone morphogenic protein and other osteoblastic genes was increased in aorta and cultured vascular smooth muscle, and importantly, these changes in gene expression preceded the development of vascular calcification in S100A12/ApoE-null mice. Accelerated atherosclerosis and vascular calcification were mediated, at least in part, by oxidative stress because inhibition of NADPH oxidase attenuated S100A12-mediated osteogenesis in cultured vascular smooth muscle cells. S100A12 transgenic mice in the wild-type background (ApoE+/+) showed minimal vascular calcification, suggesting that S100A12 requires a proinflammatory/proatherosclerotic environment to induce osteoblastic differentiation and vascular calcification.

CONCLUSIONS

Vascular smooth muscle S100A12 accelerates atherosclerosis and augments atherosclerosis-triggered osteogenesis, reminiscent of features associated with plaque instability.

PKA-induced receptor activator of NF-kappaB ligand (RANKL) expression in vascular cells mediates osteoclastogenesis but not matrix calcification

Vascular calcification is a predictor of cardiovascular mortality and is prevalent in patients with atherosclerosis and chronic renal disease. It resembles skeletal osteogenesis, and many bone cells as well as bone-related factors involved in both formation and resorption have been localized in calcified arteries. Previously, we showed that aortic medial cells undergo osteoblastic differentiation and matrix calcification both spontaneously and in response to PKA agonists. The PKA signaling pathway is also involved in regulating bone resorption in skeletal tissue by stimulating osteoblast-production of osteoclast regulating cytokines, including receptor-activator of nuclear κB ligand (RANKL) and interleukins. Therefore, we investigated whether PKA activators regulate osteoclastogenesis in aortic smooth muscle cells (SMC). Treatment of murine SMC with the PKA agonist forskolin stimulated RANKL expression at both mRNA and protein levels. Forskolin also stimulated expression of interleukin-6 but not osteoprotegerin (OPG), an inhibitor of RANKL. Consistent with these results, osteoclastic differentiation was induced when monocytic preosteoclasts (RAW264.7) were cocultured with forskolin-treated aortic SMC. Oxidized phospholipids also slightly induced RANKL expression in T lymphocytes, another potential source of RANKL in the vasculature. Because previous studies have shown that RANKL treatment alone induces matrix calcification of valvular and vascular cells, we next examined whether RANKL mediates forskolin-induced matrix calcification by aortic SMC. RANKL inhibition with OPG had little or no effect on osteoblastic differentiation and matrix calcification of aortic SMC. These findings suggest that, as in skeletal tissues, PKA activation induces bone resorptive factors in the vasculature and that aortic SMC calcification specifically induced by PKA, is not mediated by RANKL.

Vascular smooth muscle cell differentiation to an osteogenic phenotype involves TRPM7 modulation by magnesium

Arterial calcification, common in vascular diseases, involves vascular smooth muscle cell (VSMC) transformation to an osteoblast phenotype. Clinical studies suggest that magnesium may prevent this, but mechanisms are unclear. We assessed whether increasing magnesium levels reduce VSMC calcification and differentiation and questioned the role of the Mg(2+) transporter, transient receptor potential melastatin (TRPM)7 cation channels in this process. Rat VSMCs were exposed to calcification medium in the absence and presence of magnesium (2.0 to 3.0 mmol/L) or 2-aminoethoxy-diphenylborate (2-APB) (TRPM7 inhibitor). VSMCs from mice with genetically low (MgL) or high-normal (MgH) [Mg(2+)](i) were also studied. Calcification was assessed by von Kossa staining. Expression of osteocalcin, osteopontin, bone morphogenetic protein (BMP)-2, BMP-4, BMP-7, and matrix Gla protein and activity of TRPM7 (cytosol:membrane translocation) were determined by immunoblotting. Calcification medium induced osteogenic differentiation, reduced matrix Gla protein content, and increased expression of the sodium-dependent cotransporter Pit-1. Magnesium prevented calcification and decreased osteocalcin expression and BMP-2 activity and increased expression of calcification inhibitors, osteopontin and matrix Gla protein. TRPM 7 activation was decreased by calcification medium, an effect reversed by magnesium. 2-APB recapitulated the VSMC osteoblastic phenotype in VSMCs. Osteocalcin was increased by calcification medium in VSMCs and intact vessels from MgL but not MgH, whereas osteopontin was increased in MgH, but not in MgL mice. Magnesium negatively regulates vascular calcification and osteogenic differentiation through increased/restored TRPM7 activity and increased expression of anticalcification proteins, including osteopontin, BMP-7, and matrix Gla protein. New molecular insights are provided whereby magnesium could protect against VSMC calcification.

Near-infrared fluorescent probe traces bisphosphonate delivery and retention in vivo

Bisphosphonate use has expanded beyond traditional applications to include treatment of a variety of low-bone-mass conditions. Complications associated with long-term bisphosphonate treatment have been noted, generating a critical need for information describing the local bisphosphonate-cell interactions responsible for these observations. This study demonstrates that a fluorescent bisphosphonate analogue, far-red fluorescent pamidronate (FRFP), is an accurate biomarker of bisphosphonate deposition and retention in vivo and can be used to monitor site-specific local drug concentration. In vitro, FRFP is competitively inhibited from the surface of homogenized rat cortical bone by traditional bisphosphonates. In vivo, FRFP delivery to the skeleton is rapid, with fluorescence linearly correlated with bone surface area. Limb fluorescence increases linearly with injected dose of FRFP; injected FRFP does not interfere with binding of standard bisphosphonates at the doses used in this study. Long-term FRFP retention studies demonstrated that FRFP fluorescence decreases in conditions of normal bone turnover, whereas fluorescence was retained in conditions of reduced bone turnover, demonstrating preservation of local FRFP concentration. In the mandible, FRFP localized to the alveolar bone and bone surrounding the periodontal ligament and molar roots, consistent with findings of osteonecrosis of the jaw. These findings support a role for FRFP as an effective in vivo marker for bisphosphonate site-specific deposition, turnover, and long-term retention in the skeleton.

Do cytokines induce vascular calcification by the mere stimulation of TNAP activity?

Vascular calcification occurs during aging in the general population and is increased in the intima by atherosclerosis and in the media by diabetes type 2. In both intima and media, calcification may lead to the formation of a tissue very similar if not identical to bone, with bone cells and bone marrow. Since vascular calcification is associated with cardiovascular complications, a better understanding of the inducing mechanisms could lead to the development of new therapeutic strategies. Many studies have provided evidence for a role of inflammation and inflammatory cytokines such as tumour necrosis factor (TNF)-α and interleukin (IL)-1β in the vascular calcification process. TNF-α and IL-1β have indeed been shown to stimulate in vitro the expression by vascular smooth muscle cells (VSMCs) of tissue-non specific alkaline phosphatase (TNAP), a key enzyme in the mineralization process, and to trigger the trans-differentiation of VSMCs into osteoblast-like cells, expressing the master transcription factor RUNX2. These data are however somewhat contradictory with the known inhibitory effects of inflammatory cytokines on bone formation. TNF-α for instance dramatically decreases RUNX2 RNA expression, protein stability and activity, and as a consequence, is a potent inhibitor of osteoblast differentiation and bone formation. In the present article, we propose a new hypothesis to explain this calcification paradox. We propose that cytokines block bone formation by decreasing RUNX2-mediated type I collagen production in osteoblasts, whereas they induce vascular ossification by the mere stimulation of TNAP by VSMCs, independently of RUNX2. We propose that this stimulation of TNAP in VSMCs in vitro and in vivo may be sufficient to induce the calcification of collagen fibrils, and that the absence of crystal clearance, in turn, induces the differentiation of VSMCs and/or mesenchymal stem cells into bone-forming cells, eventually leading to formation of a bone-like tissue. In case future experimental studies support this hypothesis, the early stimulatory and late inhibitory effects of inflammation on vascular calcification will have to be taken into consideration in the development of new therapeutic strategies.

Arterial and aortic valve calcification inversely correlates with osteoporotic bone remodelling: a role for inflammation

AIMS

Westernized countries face a growing burden of cardiovascular calcification and osteoporosis. Despite its vast clinical significance, the precise nature of this reciprocal relationship remains obscure. We hypothesize that cardiovascular calcification progresses with inflammation and inversely correlates with bone tissue mineral density (TMD).

METHODS AND RESULTS

Arterial, valvular, and bone metabolism were visualized using near-infrared fluorescence (NIRF) molecular imaging agents, targeting macrophages and osteogenesis. We detected significant arterial and aortic valve calcification in apoE(-/-) mice with or without chronic renal disease (CRD, 30 weeks old; n = 28), correlating with the severity of atherosclerosis. We demonstrated decreases in osteogenic activity in the femurs of apoE(-/-) mice when compared with WT mice, which was further reduced with CRD. Three-dimensional micro-computed tomography imaging of the cortical and cancellous regions of femurs quantified structural remodelling and reductions in TMD in apoE(-/-) and CRD apoE(-/-) mice. We established significant correlations between arterial and valvular calcification and loss of TMD (R(2) = 0.67 and 0.71, respectively). Finally, we performed macrophage-targeted molecular imaging to explore a link between inflammation and osteoporosis in vivo. Although macrophage burden, visualized as uptake of NIRF-conjugated iron nanoparticles, was directly related to the degree of arterial and valvular inflammation and calcification, the same method inversely correlated inflammation with TMD (R(2) = 0.73; 0.83; 0.75, respectively).

CONCLUSION

This study provides direct in vivo evidence that in arteries and aortic valves, macrophage burden and calcification associate with each other, whereas inflammation inversely correlates with bone mineralization. Thus, understanding inflammatory signalling mechanisms may offer insight into selective abrogation of divergent calcific phenomena.

The Pathogenesis and treatment of the valvulopathy of aortic stenosis: Beyond the SEAS

Fibrocalcific aortic stenosis (AS) results from an active process similar to atherosclerosis that involves basement membrane disruption, lipid deposition, inflammatory cell infiltration, and calcification. Consequently, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors (statins) have been extensively studied as potential therapeutic agents capable of slowing the progression of AS. However, two randomized trials, SALTIRE and the SEAS study, showed no benefit with statin therapy for AS. These results have shed doubt over the efficacy of statin therapy for AS, although their potential efficacy at early stages of aortic valve disease remains possible. In this article, we review the pathophysiology of fibrocalcific AS and discuss future directions for its nonsurgical management in the post-SEAS era.

Bone-seeking probes for optical and magnetic resonance imaging

In clinical practice the imaging of bone tissue is based almost exclusively on x-ray or radiochemical methods. Alternative methods, such as MRI and optical imaging, can provide not only anatomical, but also physiological information, due to their ability to reflect the properties of body fluids (temperature, pH and concentration of ions). In this article we review bone targeting probes for MRI and fluorescence imaging. As bone targeting is mainly associated with phosphonate and bisphosphonate derivatives, we also focus on their sorption behavior. Also discussed in detail is the limitation of using bone-targeting probes for MRI and optical imaging mainly due to their long-time retention in bone tissue and the low permeability of tissues for light.

Chronic mineral dysregulation promotes vascular smooth muscle cell adaptation and extracellular matrix calcification

In chronic kidney disease (CKD) vascular calcification occurs in response to deranged calcium and phosphate metabolism and is characterized by vascular smooth muscle cell (VSMC) damage and attrition. To gain mechanistic insights into how calcium and phosphate mediate calcification, we used an ex vivo model of human vessel culture. Vessel rings from healthy control subjects did not accumulate calcium with long-term exposure to elevated calcium and/or phosphate. In contrast, vessel rings from patients with CKD accumulated calcium; calcium induced calcification more potently than phosphate (at equivalent calcium-phosphate product). Elevated phosphate increased alkaline phosphatase activity in CKD vessels, but inhibition of alkaline phosphatase with levamisole did not block calcification. Instead, calcification in CKD vessels most strongly associated with VSMC death resulting from calcium- and phosphate-induced apoptosis; treatment with a pan-caspase inhibitor ZVAD ameliorated calcification. Calcification in CKD vessels was also associated with increased deposition of VSMC-derived vesicles. Electron microscopy confirmed increased deposition of vesicles containing crystalline calcium and phosphate in the extracellular matrix of dialysis vessel rings. In contrast, vesicle deposition and calcification did not occur in normal vessel rings, but we observed extensive intracellular mitochondrial damage. Taken together, these data provide evidence that VSMCs undergo adaptive changes, including vesicle release, in response to dysregulated mineral metabolism. These adaptations may initially promote survival but ultimately culminate in VSMC apoptosis and overt calcification, especially with continued exposure to elevated calcium.

Optical molecular imaging in atherosclerosis

Current imaging techniques focus on evaluating the anatomical structure of blood vessel wall and atherosclerotic plaque. These techniques fail to evaluate the biological processes which take place in the vessel wall and inside the plaque. Novel imaging techniques like optical imaging can evaluate the biological and cellular processes inside the plaque and provide information which can be vital for better patient risk stratification. This review highlights the various optical imaging techniques and their application in assessing biological processes in atherosclerosis.

Relationship between smaller calcifications and lipid-rich plaques on integrated backscatter-intravascular ultrasound

We evaluated the relationship between coronary calcification and plaque characteristics using integrated backscatter-intravascular ultrasound (IB-IVUS), focusing on spotty calcification. Seventy-two patients with culprit plaques containing spotty calcification were evaluated. The average degree of all the spotty calcifications (averaged arc) negatively correlated with the % lipid volume (LV) on IB-IVUS. Multivariate analysis showed the averaged arc was an independent predictor of % LV. Our observations suggest that smaller plaque calcifications are associated with lipid-rich characteristics in patients with a spotty calcification pattern.

Imaging inflammation in real time--future of nanoparticles

The detection of subclinical early inflammation in autoimmune diseases is an important but currently technically demanding approach to direct initial diagnosis and subsequent choice of therapy. Recent advances in imaging using NP provides the potential to detect cellular recruitment, vascular activation or leakage at a subclinically stage of disease and may provide predictive "biomarkers" of future pathogenesis. The NP used are either untargeted and taken up by phagocytic cells, or are linked to a ligand, targeting localisation to the site of inflammation. Techniques, varying from MRI and fluorescence to Raman spectroscopy are being employed. In this short review, we summarise many of the recent developments in the field of NP imaging related to inflammation.

Conjoint effects of serum calcium and phosphate on risk of total, cardiovascular, and noncardiovascular mortality in the community

OBJECTIVE

Hyperphosphatemia is a cardiovascular risk factor in patients with chronic kidney disease. Relations of circulating calcium (Ca) and phosphorus (Pi) to long-term mortality risk in the community require further investigation.

METHODS AND RESULTS

Associations of serum Ca and Pi to mortality were evaluated in a community-based cohort of 2176 men (mean age, 50.1 years). During follow-up (median, 29.8 years), 1009 men died, and 466 of these deaths resulted from cardiovascular causes. In Cox proportional hazards models, serum Pi and [CaxPi] were independent predictors of total mortality (hazard ratio per SD, 1.06; 95% CI, 1.01-1.12; P=0.03; 1.07; 95% CI, 1.01-1.12; P=0.01) and cardiovascular mortality (1.10; 95% CI, 1.02-1.18; P=0.01; 1.10; 95% CI, 1.03-1.19; P=0.008). Serum Ca was associated with risk of total mortality (1.08; 95% CI, 1.01-1.16; P=0.02) and noncardiovascular mortality (1.10; 95% CI, 1.01-1.21; P=0.04). Results were consistent after multivariate adjustments in subsamples of individuals with estimated glomerular filtration rate >90 mL/min and low-to-normal serum Ca and Pi.

CONCLUSIONS

Circulating Ca and Pi levels are associated with risks of total, cardiovascular, and noncardiovascular mortality in the community, and their conjoint effects are additive. Additional studies are warranted to evaluate whether Ca and Pi are modifiable risk factors in the general population.

Pre-clinical whole-body fluorescence imaging: Review of instruments, methods and applications

Fluorescence sampling of cellular function is widely used in all aspects of biology, allowing the visualization of cellular and sub-cellular biological processes with spatial resolutions in the range from nanometers up to centimeters. Imaging of fluorescence in vivo has become the most commonly used radiological tool in all pre-clinical work. In the last decade, full-body pre-clinical imaging systems have emerged with a wide range of utilities and niche application areas. The range of fluorescent probes that can be excited in the visible to near-infrared part of the electromagnetic spectrum continues to expand, with the most value for in vivo use being beyond the 630 nm wavelength, because the absorption of light sharply decreases. Whole-body in vivo fluorescence imaging has not yet reached a state of maturity that allows its routine use in the scope of large-scale pre-clinical studies. This is in part due to an incomplete understanding of what the actual fundamental capabilities and limitations of this imaging modality are. However, progress is continuously being made in research laboratories pushing the limits of the approach to consistently improve its performance in terms of spatial resolution, sensitivity and quantification. This paper reviews this imaging technology with a particular emphasis on its potential uses and limitations, the required instrumentation, and the possible imaging geometries and applications. A detailed account of the main commercially available systems is provided as well as some perspective relating to the future of the technology development. Although the vast majority of applications of in vivo small animal imaging are based on epi-illumination planar imaging, the future success of the method relies heavily on the design of novel imaging systems based on state-of-the-art optical technology used in conjunction with high spatial resolution structural modalities such as MRI, CT or ultrasound.

Heterogeneous in vivo behavior of monocyte subsets in atherosclerosis

Monocytes and macrophages play active roles in atherosclerosis, a chronic inflammatory disease that is a leading cause of death in the developed world. The prevailing paradigm states that, during human atherogenesis, monocytes accumulate in the arterial intima and differentiate into macrophages, which then ingest oxidized lipoproteins, secrete a diverse array of proinflammatory mediators, and eventually become foam cells, the key constituents of a vulnerable plaque. Yet monocytes are heterogeneous. In the mouse, one subset (Ly-6C(hi)) promotes inflammation, expands in hypercholesterolemic conditions, and selectively gives rise to macrophages in atheromata. A different subset (Ly-6C(lo)) attenuates inflammation and promotes angiogenesis and granulation tissue formation in models of tissue injury, but its role in atherosclerosis is largely unknown. In the human, monocyte heterogeneity is preserved but it is still unresolved how subsets correspond functionally. The contradistinctive properties of these cells suggest commitment for specific function before infiltrating tissue. Such commitment argues for discriminate targeting of deleterious subsets while sparing host defense and repair mechanisms. In addition to advancing our understanding of atherosclerosis, the ability to target and image monocyte subsets would allow us to evaluate drugs designed to selectively inhibit monocyte subset recruitment or function, and to stratify patients at risk for developing complications such as myocardial infarction or stroke. In this review we summarize recent advances of our understanding of the behavioral heterogeneity of monocytes during disease progression and outline emerging molecular imaging approaches to address key questions in the field.

Chronic Hypoxia Activates the Akt and β-Catenin Pathways in Human Macrophages

Objective— Macrophage activation contributes importantly to the pathogenesis of inflammatory diseases including atherosclerosis. Macrophages exist chronically under moderate hypoxia (2% to 5% O 2 ) in inflamed tissues such as atherosclerotic plaques. However, macrophage phenotypes in such environments remain incompletely understood. This study tested the hypothesis that chronic moderate hypoxia induces macrophage activation and explored the underlying mechanisms.

Methods and Results— We cultured primary human macrophages derived from peripheral blood monocytes in moderate hypoxia (2% O 2 tension) or normoxia (21% O 2 ) for 10 days. Moderate hypoxia did not affect macrophage differentiation assessed via expression levels of scavenger receptor A. Chronic moderate hypoxia, but not normoxia, activated Akt and inactivated GSK-3β, a negative effector of Akt, thus allowing nuclear translocation of β-catenin. 2% O 2 tension increased accumulation of hypoxia-inducible factors 1α (HIF-1α) transiently at 3 to 5 days. Hypoxia induced mRNA expression of the β-catenin-associated genes: MMP-7, CD44, and c-Myc. RNAi of TCF7L2, a cofactor of β-catenin, suppressed MMP-7 expression induced by hypoxia. Inhibition of Akt phosphorylation with LY294002 abolished hypoxia-induced GSK-3β inactivation, β-catenin activation, and MMP-7 expression. Macrophages under hypoxia were more resistant for oxLDL-induced apoptosis. Moreover, phospho-Akt colocalized with MMP-7 and CD44 expression in macrophages of human atherosclerotic plaques.

Conclusions— Chronic moderate hypoxia induces macrophage activation via the Akt and β-catenin pathways, providing new insight into the pathogenesis of inflammatory diseases.

Calcification in atherosclerosis

Coronary calcification has long been known to occur as a part of the atherosclerotic process, although whether it is a marker of plaque stability or instability is still a topic of considerable debate. Coronary calcification is an active process resembling bone formation within the vessel wall and, with the advances in CT technology of the past decade, can be easily quantified and expressed as a coronary artery calcium (CAC) score. The extent of calcium is thought to reflect the total coronary atherosclerotic burden, which has generated interest in using CAC as a marker of risk of cardiovascular events. The current consensus is that large amounts of CAC identify a highly vulnerable patient rather than a vulnerable plaque or vulnerable vessel. Indeed, CAC has incremental prognostic value beyond traditional risk factors in various subsets of the population. Furthermore, whereas the presence of CAC is associated with increased risk, a zero CAC score predicts excellent short-term to mid-term prognosis, even in high-risk patients. The advent of CT angiography has perhaps clouded the importance of CAC as a long-term marker of risk, as opposed to the presence of luminal stenoses that are associated with a more immediate risk of events.

Cardiac CT in asymptomatic patients at risk

Atherosclerosis is a systemic process that can develop as early as the second or third decade of life. A significant percentage of patients who experience acute coronary syndromes often have non-obstructive coronary artery disease and thus cannot be diagnosed by their symptoms or conventional functional stress testing. It has been proposed that early detection of atherosclerosis would generate novel opportunities for primary prevention through changes in lifestyle or even through drug therapy, especially in patients at high cardiovascular risk. Calcium scoring is a simple, reproducible, and widely available test that has been extensively validated over the past two decades. In this article, the utility of calcium scoring and the potential application of CT coronary angiography in asymptomatic patients at risk are reviewed.

Mechanisms linking osteoporosis with cardiovascular calcification

Cardiovascular calcium deposition is associated with osteoporosis through various potential mechanisms involving molecular regulatory factors at the nanoscale level that govern skeletal bone and cardiovascular tissues. In this article, several possible mechanisms linking cardiovascular calcification and osteoporosis are discussed, including aging, tissue-specific responses to chronic inflammation, flow-limiting atherosclerosis of skeletal end arteries causing ischemic abnormalities in metabolism, shared endogenous regulatory factors that affect the two tissues in a reciprocal manner, and changes in a cysteine protease inhibitor, fetuin. Any or all of these factors and phenomena may contribute to the association.

Osteoporosis treatment and progression of aortic stenosis

A decrease in bone mineral density has been reported to be associated with increased progression of aortic stenosis (AS). We hypothesized that osteoporosis treatment (OT) is associated with decreased progression of AS. We performed an observational study of patients with AS from our echocardiographic database comparing 18 patients on OT (bisphosphonates, calcitonin, or estrogen receptor modulators) with 37 patients not on OT. All patients had serial echocardiograms. Patients with mitral stenosis, aortic valve replacement, renal failure, calcium disorders, or left ventricular ejection fraction <40% were excluded. Aortic valve area (AVA) was calculated using the continuity equation. There was no significant difference in age, gender, renal function, hypertension, statin use, diabetes, or calcium level between the 2 groups. Mean baseline AVA was 1.33 cm(2) and not significantly different between groups. After a mean of 2.4 +/- 1.0 years, mean annual changes in AVA were -0.22 +/- 0.22 cm(2) in those not on OT and -0.10 +/- 0.18 cm(2) in patients receiving OT (p = 0.025). There was a graded association between AS progression rate and OT. In a multivariable analysis including age, gender, and statin use, only OT was associated with a change in AVA. In conclusion, OT is strongly and independently associated with decreased progression of AS. This association warrants investigation in a larger, prospective study.