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Method Article
* Wspomniani autorzy wnieśli do projektu równy wkład.
Vascular calcification is an important predictor of and contributor to human cardiovascular disease. This protocol describes methods for inducing calcification of cultured primary vascular smooth muscle cells and for quantifying calcification and macrophage burden in animal aortas using near-infrared fluorescence imaging.
Cardiovascular disease is the leading cause of morbidity and mortality in the world. Atherosclerotic plaques, consisting of lipid-laden macrophages and calcification, develop in the coronary arteries, aortic valve, aorta, and peripheral conduit arteries and are the hallmark of cardiovascular disease. In humans, imaging with computed tomography allows for the quantification of vascular calcification; the presence of vascular calcification is a strong predictor of future cardiovascular events. Development of novel therapies in cardiovascular disease relies critically on improving our understanding of the underlying molecular mechanisms of atherosclerosis. Advancing our knowledge of atherosclerotic mechanisms relies on murine and cell-based models. Here, a method for imaging aortic calcification and macrophage infiltration using two spectrally distinct near-infrared fluorescent imaging probes is detailed. Near-infrared fluorescent imaging allows for the ex vivo quantification of calcification and macrophage accumulation in the entire aorta and can be used to further our understanding of the mechanistic relationship between inflammation and calcification in atherosclerosis. Additionally, a method for isolating and culturing animal aortic vascular smooth muscle cells and a protocol for inducing calcification in cultured smooth muscle cells from either murine aortas or from human coronary arteries is described. This in vitro method of modeling vascular calcification can be used to identify and characterize the signaling pathways likely important for the development of vascular disease, in the hopes of discovering novel targets for therapy.
Cardiovascular disease is the leading cause of morbidity and mortality in the world, including the United States where it accounts for over 780,000 deaths annually.1 Coronary artery calcification and aortic calcification are hallmarks of atherosclerotic disease and serve as strong predictors of cardiovascular events.2-4 Two main types of vascular calcification have been reported in adults: intimal calcification, associated with atherosclerosis, and medial (also known as Mönckeberg) calcification, associated with chronic kidney disease and diabetes.5 Intimal calcification occurs in the setting of lipid accumulation and macrophage infiltration into the vessel wall.5,6 Medial wall calcification occurs independently of intimal calcification, localizes to elastin fibers or smooth muscle cells, and is not associated with lipid deposition or macrophage infiltration.5,7,8 Studies on the molecular mechanisms of vascular calcification have relied on cell-based and animal model systems. Rodent models for atherocalcific disease include mice deficient in either apolipoprotein E (ApoE)9,10 or low-density lipoprotein receptor (LDLR)11 fed a high-fat diet, while models for medial calcification include mice with matrix Gla protein (MGP) deficiency12 or rats that develop uremia either by near total nephrectomy (the 5/6th nephrectomy model) or by exposure to a high-adenine diet.13
Here, the model of medial vascular calcification associated with MGP deficiency is focused on. MGP is an extracellular protein that inhibits arterial calcification.12 Mutations in the MGP gene have been identified in Keutel syndrome, a rare human disease characterized by diffuse cartilage calcification in addition to brachytelephalangy, hearing loss, and peripheral pulmonary stenosis.14-18 Although not often observed,19 concentric calcification of multiple arteries has been described in Keutel syndrome.20 Common polymorphisms in the human MGP gene are associated with increased risk for coronary artery calcification,21-23 while higher circulating levels of uncarboxylated, biologically inactive MGP predict cardiovascular mortality.24 Unlike humans with Keutel syndrome, MGP-deficient mice develop a severe vascular phenotype consisting of spontaneous widespread arterial calcification starting at two weeks of age and die 6-8 weeks after birth due to aortic rupture.12
Unlike ApoE-/- and LDLR-/- mice fed a high-fat diet, which develop intimal vascular calcification with associated macrophage-induced inflammation, MGP-/- mice develop medial vascular calcification in the absence of macrophage infiltration.11,25 Although these findings suggest different underlying stimuli for intimal and medial calcification, there is overlap in the signaling mechanisms that mediate both forms of calcification.26 Multiple signaling pathways have been identified that contribute to vascular calcification including inflammatory mediators such as tumor necrosis factor-α and IL-1 and pro-osteogenic factors such as Notch, Wnt, and bone morphogenetic protein (BMP) signaling.27,28 These signaling pathways increase expression of the transcription factors runt-related transcription factor 2 (Runx2) and osterix, which in turn increase expression of bone-related proteins (e.g., osteocalcin, sclerostin, and alkaline phosphatase) in the vasculature that mediate calcification.28-30 We and others have demonstrated that the vascular calcification observed in ApoE-/- and LDLR-/- mice fed a high-fat diet and the spontaneous vascular calcification observed in MGP-/- mice all depend on bone morphogenetic protein (BMP) signaling, and it is this pathway that is focused on here.11,25,31 BMPs are potent osteogenic factors required for bone formation and are known to exhibit increased expression in human atherosclerosis.32-34In vitro studies have implicated BMP signaling in regulating the expression of osteogenic factors such as Runx2.35-37 Overexpression of the BMP ligand, BMP-2, accelerates the development of vascular calcification in ApoE-deficient mice fed a high fat diet.38 Moreover, the use of specific BMP signaling inhibitors such as LDN-193189 (LDN)39,40 and/or ALK3-Fc prevents the development of vascular calcification in both LDLR-/- mice fed a high-fat diet and MGP-deficient mice.11,25
Vascular smooth muscle cells (VSMCs) have a critical role in the development of vascular calcification.30,41,42 The medial vascular calcification that develops in MGP-deficient mice is characterized by a transdifferentiation of VSMCs to an osteogenic phenotype. Loss of MGP results in decreased expression of VSMC markers including myocardin and alpha smooth muscle actin, with a concomitant rise in osteogenic markers such as Runx2 and osteopontin. These changes coincide with the development of vascular calcification.25,43,44
Aortic calcification and inflammation in mice are typically assessed utilizing histochemical techniques such as alkaline phosphatase activity for early calcification and osteogenic activity, von Kossa and Alizarin red staining for late calcification, and immunohistochemical protocols that target macrophage protein markers (e.g., CD68, F4/80, Mac-1, Mac-2, Mac-3).9,45 However, these standard imaging techniques require processing of aortic tissues into cross-sections, which is time consuming and imperfect due to sampling bias, and are limited in their ability to quantify inflammation and calcification in the whole aorta. This protocol describes a method to visualize and quantify whole aortic and medium-sized arterial calcification and macrophage accumulation utilizing near-infrared fluorescent (NIR) molecular imaging ex vivo. Also provided is a method for harvesting and culturing primary aortic VSMCs from mice and inducing the calcification of murine and human VSMCs in vitro in order to determine the molecular mechanisms underlying vascular calcification. These techniques provide the investigator with both in vivo and in vitro methods for studying atherocalcific disease.
All studies with mice were performed in strict accordance with the recommendations in the Guide for the Care and Use of Laboratory Animals of the National Institutes of Health. Housing and all procedures involving mice described in this study were approved by the Institutional Animal Care and Use Committees of Massachusetts General Hospital (Subcommittee on Research Animal Care). All procedures were performed with care to minimize suffering.
1. Preparation of Reagents
2. Tail Vein Injection
3. Mouse Dissection
4. Aortic Imaging
5. Isolation of Primary Murine Aortic Vascular Smooth Muscle Cells
6. Inducing Calcification of Cultured Smooth Muscle Cells
7. Assessing VSMC Calcification Using the von Kossa Staining Method
Note: The von Kossa method for measuring extracellular matrix calcification of tissues or cultured cells is based on the substitution of phosphate-bound calcium ions with silver ions.50 In the presence of light and organic compounds, the silver ions are reduced and visualized as metallic silver. Any unreacted silver is removed by treatment with sodium thiosulfate.50 The protocol for von Kossa staining is as follows:
OR
8. Assessing VSMC Calcification with Near-infrared Fluorescent Imaging
Note: Similar to its ability to identify calcification within mouse aortas, calcium NIR readily binds calcific mineral deposited by cultured cells. Using this technique, fluorescent microscopy and plate readers with long wavelength filters can image and quantify in vitro calcification, respectively. The long wavelength emission of the calcium NIR allows for the simultaneous utilization of lower wavelength emitting fluorophores to detect other features. The protocol for calcium NIR staining is as follows:
Aortic calcification in MGP-/- and wild-type mice was measured using imaging of calcium NIR fluorescence. No calcium NIR signal was detected in the aortas from wild-type mice, indicating the absence of calcification (Figure 2). A strong calcium NIR signal was detected in the aortas from MGP-deficient mice, which is consistent with advanced vascular calcification. Tissue sections of aortas from wild-type and MGP-/- mice were stained with Alizarin red<...
Arterial calcification is an important risk factor for cardiovascular disease in humans and may contribute directly to the pathogenesis of cardiovascular events.1,5,52 Intimal calcium deposition in the thin fibrous caps of atherosclerotic disease has been proposed to increase local biomechanical stress and contribute to plaque rupture.53,54 Medial calcification impacts clinical outcomes by increasing arterial stiffness, which can induce cardiac hypertrophy and affect cardiac function.55 T...
Massachusetts General Hospital has applied for patents related to small molecule inhibitors of BMP type I receptors and the application of ALK3-Fc to treat atherosclerosis and vascular calcification, and MD, PBY, KDB, and RM may be entitled to royalties.
This work was supported by the Sarnoff Cardiovascular Research Foundation (MFB and TET), the Howard Hughes Medical Institute (TM), the Ladue Memorial Fellowship Award from Harvard Medical School (DKR), the START-Program of the Faculty of Medicine at RWTH Aachen (MD), the German Research Foundation (DE 1685/1-1, MD), the National Eye Institute (R01EY022746, ESB), the Leducq Foundation (Multidisciplinary Program to Elucidate the Role of Bone Morphogenetic Protein Signaling in the Pathogenesis of Pulmonary and Systemic Vascular Diseases, PBY, KDB, and DBB), the National Institute of Arthritis and Musculoskeletal and Skin Diseases (R01AR057374, PBY), the National Institute of Diabetes and Digestive and Kidney Diseases (R01DK082971, KDB and DBB), the American Heart Association Fellow-to-Faculty Award #11FTF7290032 (RM), and the National Heart, Lung, and Blood Institute (R01HL114805 and R01HL109506, EA; K08HL111210, RM).
Name | Company | Catalog Number | Comments |
15 ml conical tube | Falcon | 352096 | |
30 G needle | BD | 305106 | |
Alpha smooth muscle actin antibody | Sigma | SAB2500963 | |
Chamber slide | Nunc Lab-Tek | 154461 | |
Collagenase, Type 2 | Worthington | LS004176 | |
Dexamethasone | Sigma | D4902 | |
Dulbecco's Modified Eagle Medium | Life Technologies | 11965-084 | |
Dulbecco's Phosphate Buffered Saline, no calcium | Gibco | 14190-144 | |
Elastase | Sigma | E1250 | |
Fetal bovine serum | Gibco | 16000-044 | |
Forceps, fine point | Roboz | RS-4972 | |
Forceps, full curve serrated | Roboz | RS-5138 | |
Formalin (10%) | Electron Microscopy Sciences | 15740 | |
Hank's Balanced Salt Solution | Gibco | 14025-092 | |
Human coronary artery smooth muscle cells | PromoCell | C-12511 | |
Insulin syringe with needle | Terumo | SS30M2913 | |
L-ascorbic acid | Sigma | A-7506 | |
Micro-dissecting spring scissors (13 mm) | Roboz | RS-5676 | |
Micro-dissecting spring scissors (3 mm) | Roboz | RS-5610 | |
NIR, cathepsin (ProSense-750EX) | Perkin Elmer | NEV10001EX | |
NIR, osteogenic (OsteoSense-680EX) | Perkin Elmer | NEV10020EX | |
Normal Saline | Hospira | 0409-4888-10 | |
Nuclear fast red | Sigma-Aldrich | N3020 | |
Odyssey Imaging System | Li-Cor | Odyssey 3.0 | |
Penicillin/Streptomycin | Corning | 30-001-CI | |
Silver nitrate (5%) | Ricca Chemical Company | 6828-16 | |
Sodium phosphate dibasic heptahydrate | Sigma-Aldrich | S-9390 | |
Sodium thiosulfate | Sigma | S-1648 | |
ß-glycerophosphate disodium salt hydrate | Sigma | G9422 | |
Tissue culture flask, 25 cm2 | Falcon | 353108 | |
Tissue culture plate (35 mm x 10 mm) | Falcon | 353001 | |
Tissue culture plate, six-well | Falcon | 353046 | |
Trypsin | Corning | 25-053-CI | |
Tube rodent holder | Kent Scientific | RSTR551 | |
Vacuum-driven filtration system | Millipore | SCGP00525 |
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