SciCombinator

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Concept: Calcification

147

Vascular calcification, occurring during late-stage vascular and valvular disease, is highly associated with chronic kidney disease-mineral and bone disorders (CKD-MBD), representing a major risk factor for cardiovascular morbidity and mortality. The hallmark of vascular calcification, which involves both media and intima, is represented by the activation of cells committed to an osteogenic programme. Several studies have analysed the role of circulating calcifying cells (CCCs) in vascular calcification. CCCs are bone marrow (BM)-derived cells with an osteogenic phenotype, participating in intima calcification processes and defined by osteocalcin and bone alkaline phosphatase expression. The identification of CCCs in diabetes and atherosclerosis is the most recent, intriguing and yet uncharted chapter in the scenario of the bone-vascular axis. Whether osteogenic shift occurs in the BM, the bloodstream or both, is not known, and also the factors promoting CCC formation have not been identified. However, it is possible to recognize a common pathogenic commitment of inflammation in atherosclerosis and diabetes, in which metabolic control may also have a role. Currently available studies in patients without CKD did not find an association of CCCs with markers of bone metabolism. Preliminary data on CKD patients indicate an implication of mineral bone disease in vascular calcification, as a consequence of functional and anatomic integrity interruption of BM niches. Given the pivotal role that parathyroid hormone and osteoblasts play in regulating expansion, mobilization and homing of haematopoietic stem/progenitors cells, CKD-MBD could promote CCC formation.

Concepts: Immune system, Bone, Cancer, Metabolism, Bone marrow, Alkaline phosphatase, Organ, Calcification

27

Vascular mineralization has recently emerged as a risk factor for cardiovascular morbidity and mortality. Previously regarded as a passive end-stage process, vascular mineralization is currently recognized as an actively regulated process with cellular and humoral contributions. The discovery that the vitamin K-dependent matrix Gla-protein (MGP) is a strong inhibitor of vascular calcification has propelled our mechanistic understanding of this process and opened novel avenues for diagnosis and treatment. This review focuses on molecular mechanisms of vascular mineralization involving MGP and discusses the potential for treatments and biomarkers to monitor patients at risk for vascular mineralization.

Concepts: Mortality rate, English-language films, Investment, American films, Calcification, Carboxylation

25

Aims: To demonstrate the feasibility of the Leaflex™ Catheter System, a novel percutaneous device for fracturing valve calcification using mechanical impact in order to regain leaflet mobility. Methods and results: Radiographic analysis of calcium patterns in 90 ex vivo human aortic valve leaflets demonstrated that 82% of leaflets had a typical “bridge” or “half-bridge” pattern, which formed the basis for the catheter design. The therapeutic effect was quantified in 13 leaflets showing a reduction of 49±16% in leaflet resistance to folding after treatment. A pulsatile flow simulator was then used with 11 ex vivo valves demonstrating an increase in aortic valve area of 35±12%. Using gross pathology and histology on fresh calcified leaflets, we then verified that mechanical impacts do not entail excessive risk of embolisation. In vivo safety and usability were then confirmed in the ovine model. Conclusions: We demonstrated preclinically that it is feasible to improve valve function using the Leaflex™ technology. Once demonstrated clinically, such an approach may have an important role as preparation for or bridging to TAVI, as destination treatment for patients where TAVI is clinically or economically questionable and, in the future, maybe even as a means to slow disease progression in asymptomatic patients.

Concepts: In vivo, Demonstration, Aortic valve, Valvular heart disease, Calcification, Aortic valve stenosis, Leaflet, Gross examination

3

Vascular calcification is a hallmark of atherosclerosis. The location, density, and confluence of calcification may change portions of the arterial conduit to a noncompliant structure. Calcifications may also seed the cap of a thin cap fibroatheroma, altering tensile forces on the cap and rendering the lesion prone to rupture. Many local and systemic factors participate in this process, including hyperlipidemia, ongoing inflammation, large necrotic cores, and diabetes. Vascular cells can undergo chondrogenic or osteogenic differentiation, causing mineralization of membranous bone and formation of endochondral bone. Calcifying vascular cells are derived from local smooth muscle cells and circulating hematopoietic stem cells (especially in intimal calcification). Matrix vesicles in the extracellular space of the necrotic core serve as a nidus for calcification. Although coronary calcification is a marker of coronary atheroma, dense calcification (>400 HU) is usually associated with stable plaques. Conversely, microcalcification (often also referred to as spotty calcification) is more commonly an accompaniment of vulnerable plaques. Recent studies have suggested that microcalcification in the fibrous cap may increase local tissue stress (depending on the proximity of one microcalcific locus to another, and the orientation of the microcalcification in reference to blood flow), resulting in plaque instability. It has been proposed that positron emission tomography imaging with sodium fluoride may identify early calcific deposits and hence high-risk plaques.

Concepts: Blood, Myocardial infarction, Atherosclerosis, Heart, Atheroma, Artery, Smooth muscle, Calcification

3

Vascular calcification is an advanced feature of atherosclerosis for which no effective therapy is available. To investigate the modulation or reversal of calcification, we identified calcifying progenitor cells and investigated their calcifying/decalcifying potentials. Cells from the aortas of mice were sorted into four groups using Sca-1 and PDGFRα markers. Sca-1(+) (Sca-1(+)/PDGFRα(+) and Sca-1(+)/PDGFRα(-)) progenitor cells exhibited greater osteoblastic differentiation potentials than Sca-1(-) (Sca-1(-)/PDGFRα(+) and Sca-1(-)/PDGFRα(-)) progenitor cells. Among Sca-1(+) progenitor populations, Sca-1(+)/PDGFRα(-) cells possessed bidirectional differentiation potentials towards both osteoblastic and osteoclastic lineages, whereas Sca-1(+)/PDGFRα(+) cells differentiated into an osteoblastic lineage unidirectionally. When treated with a peroxisome proliferator activated receptor γ (PPARγ) agonist, Sca-1(+)/PDGFRα(-) cells preferentially differentiated into osteoclast-like cells. Sca-1(+) progenitor cells in the artery originated from the bone marrow (BM) and could be clonally expanded. Vessel-resident BM-derived Sca-1(+) calcifying progenitor cells displayed nonhematopoietic, mesenchymal characteristics. To evaluate the modulation of in vivo calcification, we established models of ectopic and atherosclerotic calcification. Computed tomography indicated that Sca-1(+) progenitor cells increased the volume and calcium scores of ectopic calcification. However, Sca-1(+)/PDGFRα(-) cells treated with a PPARγ agonist decreased bone formation 2-fold compared with untreated cells. Systemic infusion of Sca-1(+)/PDGFRα(-) cells into Apoe(-/-) mice increased the severity of calcified atherosclerotic plaques. However, Sca-1(+)/PDGFRα(-) cells in which PPARγ was activated displayed markedly decreased plaque severity. Immunofluorescent staining indicated that Sca-1(+)/PDGFRα(-) cells mainly expressed osteocalcin; however, activation of PPARγ triggered receptor activator for nuclear factor-κB (RANK) expression, indicating their bidirectional fate in vivo. These findings suggest that a subtype of BM-derived and vessel-resident progenitor cells offer a therapeutic target for the prevention of vascular calcification and that PPARγ activation may be an option to reverse calcification.

Concepts: Gene expression, Atherosclerosis, Stem cell, Bone marrow, Atheroma, Cellular differentiation, Artery, Calcification

2

Clinical evidence links arterial calcification and cardiovascular risk. Finite-element modelling of the stress distribution within atherosclerotic plaques has suggested that subcellular microcalcifications in the fibrous cap may promote material failure of the plaque, but that large calcifications can stabilize it. Yet the physicochemical mechanisms underlying such mineral formation and growth in atheromata remain unknown. Here, by using three-dimensional collagen hydrogels that mimic structural features of the atherosclerotic fibrous cap, and high-resolution microscopic and spectroscopic analyses of both the hydrogels and of calcified human plaques, we demonstrate that calcific mineral formation and maturation results from a series of events involving the aggregation of calcifying extracellular vesicles, and the formation of microcalcifications and ultimately large calcification areas. We also show that calcification morphology and the plaque’s collagen content-two determinants of atherosclerotic plaque stability-are interlinked.

Concepts: Cholesterol, Myocardial infarction, Atherosclerosis, Cardiovascular disease, Low-density lipoprotein, Atheroma, Artery, Calcification

2

Vascular calcification is a complex biological process that is a hallmark of atherosclerosis. While macrocalcification confers plaque stability, microcalcification is a key feature of high-risk atheroma and is associated with increased morbidity and mortality. Positron emission tomography and X-ray computed tomography (PET/CT) imaging of atherosclerosis using (18)F-sodium fluoride ((18)F-NaF) has the potential to identify pathologically high-risk nascent microcalcification. However, the precise molecular mechanism of (18)F-NaF vascular uptake is still unknown. Here we use electron microscopy, autoradiography, histology and preclinical and clinical PET/CT to analyse (18)F-NaF binding. We show that (18)F-NaF adsorbs to calcified deposits within plaque with high affinity and is selective and specific. (18)F-NaF PET/CT imaging can distinguish between areas of macro- and microcalcification. This is the only currently available clinical imaging platform that can non-invasively detect microcalcification in active unstable atherosclerosis. The use of (18)F-NaF may foster new approaches to developing treatments for vascular calcification.

Concepts: Electron, Biology, Medical imaging, Positron emission tomography, Positron, Radiography, Radiology, Calcification

2

Seaweeds are key species of the Baltic Sea benthic ecosystems. They are the substratum of numerous fouling epibionts like bryozoans and tubeworms. Several of these epibionts bear calcified structures and could be impacted by the high pCO2 events of the late summer upwellings in the Baltic nearshores. Those events are expected to increase in strength and duration with global change and ocean acidification. If calcifying epibionts are impacted by transient acidification as driven by upwelling events, their increasing prevalence could cause a shift of the fouling communities toward fleshy species. The aim of the present study was to test the sensitivity of selected seaweed macrofoulers to transient elevation of pCO2 in their natural microenvironment, i.e. the boundary layer covering the thallus surface of brown seaweeds. Fragments of the macroalga Fucus serratus bearing an epibiotic community composed of the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium hirsutum (Bryozoa) were maintained for 30 days under three pCO2 conditions: natural 460±59 µatm, present-day upwelling1193±166 µatm and future upwelling 3150±446 µatm. Only the highest pCO2 caused a significant reduction of growth rates and settlement of S. spirorbis individuals. Additionally, S. spirorbis settled juveniles exhibited enhanced calcification of 40% during daylight hours compared to dark hours, possibly reflecting a day-night alternation of an acidification-modulating effect by algal photosynthesis as opposed to an acidification-enhancing effect of algal respiration. E. pilosa colonies showed significantly increased growth rates at intermediate pCO2 (1193 µatm) but no response to higher pCO2. No effect of acidification on A. hirsutum colonies growth rates was observed. The results suggest a remarkable resistance of the algal macro-epibionts to levels of acidification occurring at present day upwellings in the Baltic. Only extreme future upwelling conditions impacted the tubeworm S. spirorbis, but not the bryozoans.

Concepts: Algae, Photosynthesis, Atlantic Ocean, Baltic Sea, Calcification, Fucus, Seaweed

1

Coronary artery calcification (CAC) is highly prevalent among dialysis patients and is associated with increased cardiovascular and all cause mortality. Magnesium (Mg) inhibits vascular calcification in animal and in-vitro studies but whether the same effect occurs in humans is uncertain.

Concepts: Dialysis, Heart, Artery, Coronary circulation, Cardiovascular system, Calcification

1

Diseases of ectopic calcification of the vascular wall range from lethal orphan diseases such as generalized arterial calcification of infancy (GACI), to common diseases such as hardening of the arteries associated with aging and calciphylaxis of chronic kidney disease (CKD). GACI is a lethal orphan disease in which infants calcify the internal elastic lamina of their medium and large arteries and expire of cardiac failure as neonates, while calciphylaxis of CKD is a ubiquitous vascular calcification in patients with renal failure. Both disorders are characterized by vascular Mönckeburg’s sclerosis accompanied by decreased concentrations of plasma inorganic pyrophosphate (PPi). Here we demonstrate that subcutaneous administration of an ENPP1-Fc fusion protein prevents the mortality, vascular calcifications and sequela of disease in animal models of GACI, and is accompanied by a complete clinical and biomarker response. Our findings have implications for the treatment of rare and common diseases of ectopic vascular calcification.

Concepts: Chronic kidney disease, Kidney, Nephrology, Epidemiology, Death, Myocardial infarction, Prevalence, Calcification