Osteo-progenitors in vascular calcification: a circulating cell theory

Do Interactions of Vitamin D3 and BMP Signaling Hold Implications in the Pathogenesis of Fibrodysplasia Ossificans Progressiva?

PURPOSE OF REVIEW

Fibrodysplasia ossificans progressiva (FOP) is a debilitating rare disease known for episodic endochondral heterotopic ossification (HO) caused by gain-of-function mutations in ACVR1/ALK2. However, disease severity varies among patients with identical mutations suggesting disease-modifying factors, including diet, may have therapeutic implications. The roles of vitamin D₃ in calcium metabolism and chondrogenesis are known, but its effects on BMP signaling and chondrogenesis are less studied. This review attempts to assess the possibility of vitamin D's effects in FOP by exploring relevant intersections of VD₃ with mechanisms of FOP flares.

RECENT FINDINGS

In vitro and in vivo studies suggest vitamin D suppresses inflammation, while clinical studies suggest that vitamin D₃ protects against arteriosclerosis and inversely correlates with non-genetic intramuscular HO. However, the enhancement of chondrogenesis, BMP signaling, and possibly Activin A expression by vitamin D may be more relevant in FOP. There appears to be little potential for vitamin D to reduce HO in FOP, but testing the potential for excess vitamin D to promote HO may be warranted.

[The cellular composition of explanted bioprosthetic heart valves in infective endocarditis]

OBJECTIVE

To perform immunohistochemical typing of cells as a component of bioprosthetic (BP) heart valves explanted during reoperations for prosthetic valve endocarditis.

MATERIAL AND METHODS

The authors investigated 8 models of KemCor and PeriCor artificial heart valves produced by NeoCor Company (Kemerovo, Russia), which were explanted from the mitral position due to infection of xenogeneic implanted material. The following markers: CD3 (T-lymphocytes), CD20 (B-lymphocytes), CD34 and VEGFR2 (endotheliocytes), CD68 (monocytes/macrophages), vimentin (fibroblasts), and α-smooth muscle actin (smooth muscle cells), were used for immunohistochemical typing of cells as a component of the analyzed samples.

RESULTS

Recipient cells were found to colonize devitalized BP tissues in infective endocarditis. This process simultaneously involved several types of cells performing their functions in infectious lesion and its initiation of BP remodeling. Macrophages contributed to the sanitation of the foci of infection and destruction of BP xenotissue; endotheliocytes ensured neovascularization and resistance of the implanted valve surface to infection; fibroblasts played a role in the neoplastic transformation of collagen, and smooth muscle cells were likely to take on the role in forming the elastic framework of a leaflet and in ensuring the mechanical properties of the bioprosthesis.

CONCLUSION

In the time course of development of prosthetic endocarditis, the recipient cells populate xenovalve leaflets that are a modified extracellular matrix obtained from the porcine aortic valve complex. This process is a consequence of the destruction of the BP surface and deep components. The observed cellular reactions are likely to be adaptive and to be aimed at eliminating microorganisms and regenerating structural damages.

The Emerging Role of Mesenchymal Stem Cells in Vascular Calcification

Vascular calcification (VC), characterized by hydroxyapatite crystal depositing in the vessel wall, is a common pathological condition shared by many chronic diseases and an independent risk factor for cardiovascular events. Recently, VC is regarded as an active, dynamic cell-mediated process, during which calcifying cell transition is critical. Mesenchymal stem cells (MSCs), with a multidirectional differentiation ability and great potential for clinical application, play a duplex role in the VC process. MSCs facilitate VC mainly through osteogenic transformation and apoptosis. Meanwhile, several studies have reported the protective role of MSCs. Anti-inflammation, blockade of the BMP2 signal, downregulation of the Wnt signal, and antiapoptosis through paracrine signaling are possible mechanisms. This review displays the evidence both on the facilitating role and on the protective role of MSCs, then discusses the key factors determining this divergence.

Treatment with XAV-939 prevents in vitro calcification of human valvular interstitial cells

The development of a substance or inhibitor-based treatment strategy for the prevention of aortic valve stenosis is a challenge and a main focus of medical research in this area. One strategy may be to use the tankyrase inhibitor XAV-939, which leads to Axin stabilisation and subsequent destruction of the β-catenin complex and dephosphorylation of β-catenin. The dephosphorylated active form of β-catenin (non-phospho-β-catenin) then promotes nuclear transcription that leads to osteogenesis. The aims of the present study were to develop an experimental system for inducing in vitro calcification of human aortic valvular interstitial cells (VICs) to investigate the potential anti-calcific effect of XAV-939 and to analyse expression of the Wnt signalling proteins and Sox9, a chondrogenesis regulator, in this model. Calcification of human VIC cultures was induced by cultivation in an osteogenic medium and the effect of co-incubation with 1μM XAV-939 was monitored. Calcification was quantified when mineral deposits were visible in culture and was histologically verified by von Kossa or Alizarin red staining and by IR-spectroscopy. Protein expression of alkaline phosphatase, Axin, β-catenin and Sox9 were quantified by western blotting. In 58% of the VIC preparations, calcification was induced in an osteogenic culture medium and was accompanied by upregulation of alkaline phosphatase. The calcification induction was prevented by the XAV-939 co-treatment and the alkaline phosphatase upregulation was suppressed. As expected, Axin was upregulated, but the levels of active non-phospho-β-catenin were also enhanced. Sox9 was induced during XAV-939 treatment but apparently not as a result of downregulation of β-catenin signalling. XAV-939 was therefore able to prevent calcification of human VIC cultures, and XAV-939 treatment was accompanied by upregulation of active non-phospho-β-catenin. Although XAV-939 does not downregulate active β-catenin, treatment with XAV-939 results in Sox9 upregulation that may prevent the calcification process.

Potential Role for Osteocalcin in the Development of Atherosclerosis and Blood Vessel Disease

There is increasing evidence for the involvement of the skeleton in the regulation of atherosclerotic vascular disease. Osteocalcin, an osteoblast derived protein, exists in two forms, carboxylated and undercarboxylated osteocalcin. Undercarboxylated osteocalcin has been linked to the regulation of metabolic functions, including glucose and lipid metabolism. Features of atherosclerosis have been associated with circulating osteocalcin; however, this association is often conflicting and unclear. Therefore, the aim of this review is to examine the evidence for a role of osteocalcin in atherosclerosis development and progression, and in particular endothelial dysfunction and vascular calcification. The current literature suggests that undercarboxylated osteocalcin stimulates the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) signaling pathway to upregulate nitric oxide and nuclear factor kappa β (NF-кβ) in vascular cells, possibly protecting endothelial function and preventing atherogenesis. However, this effect may be mediated by metabolic factors, such as improvements in insulin signaling, rather than through a direct effect on the vasculature. Total osteocalcin is frequently associated with vascular calcification, an association that may occur as a result of vascular cells eliciting an osteogenic phenotype. Whether osteocalcin acts as a mediator or a marker of vascular calcification is currently unclear. As such, further studies that examine each form of osteocalcin are required to elucidate if it is a mediator of atherogenesis, and whether it functions independently of metabolic factors.

Serum Tartrate-resistant Acid Phosphatase-5b Levels are Associated with the Severity and Extent of Coronary Atherosclerosis in Patients with Coronary Artery Disease

Aims: Tartrate-resistant acid phosphatase (TRACP)-5b and osteoprotegerin (OPG) are specific and sensitive markers of bone resorption in patients with rheumatoid arthritis (RA) and chronic kidney disease (CKD). The TRACP-5b level is associated with the severity of RA and CKD, while the OPG level is associated with the severity of coronary atherosclerosis and calcification, and can predict a poor outcome in patients with coronary artery disease (CAD). However, the impact of TRACP-5b on coronary atherosclerosis in CAD patients remains unclear.

Methods: A total of 71 CAD patients (57 men, 14 women; mean age: 69.0 ± 9.7 years) and 28 age- and gender-matched healthy subjects were investigated. The number of diseased vessels (a marker of the severity of coronary atherosclerosis) and the Gensini score (a marker of the extent of coronary atherosclerosis), as well as the OPG and TRACP-5b levels were measured in CAD patients. The TRACP-5b levels were classified into quartiles.

Results: The TRACP-5b levels were significantly higher in CAD patients than in healthy subjects. Patients with higher TRACP-5b levels had higher OPG levels and Gensini scores than those with lower TRACP-5b levels. Higher TRACP-5b levels were associated with an increased number of diseased vessels. A multivariate linear regression analysis showed that the OPG level and the number of diseased vessels or the Gensini score were significantly and independently associated with the TRACP-5b level.

Conclusions: These data indicate that the TRACP-5b level is significantly associated with the OPG level and with the severity and extent of coronary atherosclerosis in CAD patients.

[Investigation of the structure of a functionally intact xenopericardial bioconduit after long-term implantation]

AIM

to investigate the cellular composition of a functionally intact xenopericardial valve in a recipient with acquired mitral defect after long-term implantation.

MATERIAL AND METHODS

A Uniline bioconduit (BC) ('Neocor', Kemerovo) removed from the heart in the mitral position at 7.2 years after implantation was investigated. Heart valve leaflets were fixed in a buffered 4% paraformaldehyde solution and imbedded in paraffin or epoxy resin. Slices made from the paraffin samples were stained with hematoxylin and eosin or underwent immunohistochemical (IHC) examination for typing endothelial cells, smooth muscle cells, macrophages, fibroblasts, and T and B lymphocytes. The epoxy resin-embedded samples were examined using light and scanning electron microscopy according to the original procedure. For this, the samples were ground and polished, then stained with toluidine blue and basic fuchsin or contrasted with uranyl acetate and lead citrate.

RESULTS

Different cell types were found in the outer layers of heart valve leaflets. IHC showed that endothelial cells, macrophages, smooth muscle cells, and fibroblasts were present in the samples. A relationship was found between the degree of degenerative changes in the BC surface and the magnitude of cellular infiltration in xenotissue. This paper debates whether impaired integrity of the surface leaflet layers plays a trigger role in structural dysfunctions of the implanted valves and whether BC endothelialization has a protective effect, which can considerably reduce the immunogenicity of xenotussie and prevent the penetration of recipient cells.

CONCLUSION

The paper shows that it is expedient to modify the surface of the heart valve leaflets in order to create favorable conditions for the attachment and function of endothelial progenitor cells.

The study of calcified atherosclerotic arteries: an alternative to evaluate the composition of a problematic tissue reveals new insight including metakaryotic cells

Background

Calcifications of atherosclerotic plaques represent a controversial issue as they either lead to the stabilization or rupture of the lesion. However, the cellular key players involved in the progression of the calcified plaques have not yet been described. The primary reason for this lacuna is that decalcification procedures impair protein and nucleic acids contained in the calcified tissue. The aim of our study was to preserve the cellular content of heavily calcified plaques with a new rapid fixation in order to simplify the study of calcifications.

Methods

Here we applied a fixation method for fresh calcified tissue using the Carnoy’s solution followed by an enzymatic tissue digestion with type II collagenase. Immunohistochemistry was performed to verify the preservation of nuclear and cytoplasmic antigens. DNA content and RNA preservation was evaluated respectively with Feulgen staining and RT-PCR. A checklist of steps for successful image analysis was provided. To present the basic features of the F-DNA analysis we used descriptive statistics, skewness and kurtosis. Differences in DNA content were analysed with Kruskal-Wallis and Dunn’s post tests. The value of P < 0.05 was considered significant.

Results

Twenty-four vascular adult tissues, sorted as calcified (14) or uncalcified (10), were processed and 17 fetal tissues were used as controls (9 soft and 8 hard). Cells composing the calcified carotid plaques were positive to Desmin, Vimentin, Osteocalcin or Ki-67; the cellular population included smooth muscle cells, osteoblasts and osteoclasts-like cells and metakaryotic cells. The DNA content of each cell type found in the calcified carotid artery was successfully quantified in 7 selected samples. Notably the protocol revealed that DNA content in osteoblasts in fetal control tissues exhibits about half (3.0 ng) of the normal nuclear DNA content (6.0 ng).

Conclusion

Together with standard histology, this technique could give additional information on the cellular content of calcified plaques and help clarify the calcification process during atherosclerosis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12907-016-0036-6) contains supplementary material, which is available to authorized users.

Indirect co‑culture of vascular smooth muscle cells with bone marrow mesenchymal stem cells inhibits vascular calcification and downregulates the Wnt signaling pathways

Vascular calcification (VC) is widely considered to be a crucial clinical indicator of cardiovascular disease. Recently, certain properties of mesenchymal stem cells (MSCs) have been hypothesized to have potential in treating cardiovascular diseases. However, their effect on the initiation and progression of VC remains controversial. The present study aimed to investigate whether MSCs indirectly mediate VC and their impact on the Wnt signaling pathways. A Transwell system was selected to establish the indirect co‑culture environment, and hence, vascular smooth muscle cells (VSMCs) were indirectly co‑cultured in the presence or absence of MSCs at a ratio of 1:1. Osteogenic medium (OS) was added to imitate a calcifying environment. Fourteen days later, VSMCs in the lower layers of the Transwell plates were harvested. Alkaline phosphatase activity and calcium nodules were markedly increased in calcific VSMCs induced by OS. However, these parameters were significantly decreased in VSMCs by indirectly co‑culturing with MSCs in the same medium. Furthermore, the messenger RNA expression levels of osteopontin and osteoprotegerin were notably increased in VSMCs cultured in OS, but reduced by indirect interaction with MSCs. In addition, the activities of canonical and noncanonical Wnt ligands, wingless‑type MMTV integration site family, number 5A (Wnt5a), receptor tyrosine kinase‑like orphan receptor 2 (Ror2) and β‑catenin, which are important in the process of VC, were downregulated by indirect contact with MSCs in OS. Thus, indirect co‑culture with MSCs inhibits VC and downregulates the Wnt signaling pathways.

Vascular calcification: from pathophysiology to biomarkers

The link between vascular calcification (VC) and increased mortality is now well established. Over time, as clinical importance of this phenomenon has begun to be fully considered, scientists have highlighted more and more physiopathological mechanisms and signaling pathways that underlie VC. Several conditions such as diabetes, dyslipidemia and renal diseases are undoubtedly identified as predisposing factors. But even if the process is better understood, many questions still remain unanswered. This review briefly develops the various theories that attempt to explain mineralization genesis. Nonetheless, the main purpose of the article is to provide a profile of the various existing biomarkers of VC. Indeed, in the past years, a lot of inhibitors and promoters, which form a dense and interconnected network, were identified. Given importance to assess and control mineralization process, a focusing on accumulated knowledge of each marker seemed to be necessary. Therefore, we tried to define their respective role in the physiopathology and how they can contribute to calcification risk assessment. Among these, Klotho/fibroblast growth factor-23, fetuin-A, Matrix Gla protein, Bone morphogenetic protein-2, osteoprotegerin, osteopontin, osteonectin, osteocalcin, pyrophosphate and sclerostin are specifically discussed.

Arterial calcification: Finger-pointing at resident and circulating stem cells

The term ‘‘Stammzelle’’ (stem cells) originally appeared in 1868 in the works of Ernst Haeckel who used it to describe the ancestor unicellular organism from which he presumed all multicellular organisms evolved. Since then stem cells have been studied in a wide spectrum of normal and pathological conditions; it is remarkable to note that ectopic arterial calcification was considered a passive deposit of calcium since its original discovering in 1877; in the last decades, resident and circulating stem cells were imaged to drive arterial calcification through chondro-osteogenic differentiation thus opening the idea that an active mechanism could be at the basis of the process that clinically shows a Janus effect: calcifications either lead to the stabilization or rupture of the atherosclerotic plaques. A review of the literature underlines that 130 years after stem cell discovery, antigenic markers of stem cells are still debated and the identification of the osteoprogenitor phenotype is even more elusive due to tissue degradation occurring at processing and manipulation. It is necessary to find a consensus to perform comparable studies that implies phenotypic recognition of stem cells antigens. A hypothesis is based on the singular morphology and amitotic mechanism of division of osteoclasts: it constitutes the opening to a new approach on osteoprogenitors markers and recognition. Our aim was to highlight all the present evidences of the active calcification process, summarize the different cellular types involved, and discuss a novel approach to discover osteoprogenitor phenotypes in arterial wall.

Mesenchymal stem cells recruited by active TGFβ contribute to osteogenic vascular calcification

Vascular calcification is an actively regulated process that culminates in organized extracellular matrix mineral deposition by osteoblast-like cells. The origins of the osteoblastic cells involved in this process and the underlying mechanisms remain to be defined. We previously revealed that active transforming growth factor (TGFβ) released from the injured arteries mobilizes mesenchymal stem cells (MSCs) to the blood stream and recruits the cells to the injured vessels for neointima formation. In this study, we used a low-density lipoprotein receptor (LDLR)-deficient mouse model (ldlr(-/-)), which develop progressive arterial calcification after having fed high-fat western diets (HFD), to examine whether TGFβ is involved in the mobilization of MSCs during vascular calcification. Nestin(+)/Sca1(+) cells were recruited to the diseased aorta at earlier time points, and osteocalcin(+) osteoblasts and the aortic calcification were seen at later time point in these mice. Importantly, we generated parabiotic pairs with shared blood circulation by crossing ldlr(-/-)mice fed HFD with transgenic mice, in which all the MSC-derived cells were fluorescently labeled. The labeled cells were detected not only in the peripheral blood but also in the arterial lesions in ldlr(-/-) mouse partners, and these blood circulation-originated cells gave rise to Ocn(+) osteoblastic cells at the arterial lesions. Both active TGFβ1 levels and MSCs in circulating blood were upregulated at the same time points when these cells appeared at the aortic tissue. Further, conditioned medium prepared by incubating the aortae from ldlr(-/-)mice fed HFD stimulated the migration of MSCs in the ex vivo transwell assays, and either TGFβ neutralizing antibody or the inhibitor of TGFβ Receptor I kinase (TβRI) antagonized this effect. Importantly, treatment of the mice with TβRI inhibitor blocked elevated blood MSC numbers and their recruitment to the arterial lesions. These findings suggest that TGFβ-recruited MSCs to the diseased vasculature contribute to the development of osteogenic vascular calcification.

Cytotoxic Effects of Polyhexanide on Cellular Repopulation and Calcification of Decellularized Equine Carotids in vitro and in vivo

Purpose

Disinfection of biological implants is indispensable for clinical safety. Here, decellularized equine carotid arteries (dECAs) were disinfected by polyhexanide (PHX), an effective, well-tolerated and nontoxic wound disinfectant and evaluated as vascular grafts for their repopulation and local biocompatibility in vivo.

Methods

dECAs were terminally disinfected by a combination of 0.1% PHX and 70% ethanol (dECA_PHX-ET) or exclusively ethanol (dECA-ET) and subsequently implanted as arteriovenous shunts in sheep for 14 weeks. Repopulation was determined by immunohistochemistry for endothelial- (ECs) or smooth muscle cells (SMCs) using antibodies against CD31 and smooth muscle actin. Histological evaluation was performed on HE-stained sections. Cytotoxicity of dECAs was measured directly by seeding the scaffolds with L-929 fibroblasts, which were visualized by calcein staining. Indirect cytotoxicity was determined by WST-8 viability assay by incubation of L-929 with dECA extracts.

Results

dECA_PHX-ET completely lacked repopulation with ECs and SMCs, showed leukocyte infiltration, strong calcification and poor neovascularization indicating insufficient biocompatibility and inflammatory graft degeneration. PHX-treatment reduced cell viability to 33.2 ± 12.6% and disturbed cell growth at direct contact. In contrast, dECA_ET had no direct cytotoxic effect and only slightly influenced cell viability (82.9 ± 12.5%), showed a substantial repopulation by ECs and SMCs including neovascularization, and were only slightly calcified.

Conclusion

The disinfectant polyhexanide seems to exert severe cytotoxic effects when used for the processing of decellularized matrices and may result in degenerative graft deterioration. In contrast, dECAs exclusively disinfected with ethanol were well integrated. Thus, ethanol seems to be a more suitable tool for graft processing than polyhexanide.