Role for circulating osteogenic precursor cells in aortic valvular disease

Cell Senescence in Heterotopic Ossification

The formation of bone outside the normal skeleton, or heterotopic ossification (HO), occurs through genetic and acquired mechanisms. Fibrodysplasia ossificans progressiva (FOP), the most devastating genetic condition of HO, is due to mutations in the ACVR1/ALK2 gene and is relentlessly progressive. Acquired HO is mostly precipitated by injury or orthopedic surgical procedures but can also be associated with certain conditions related to aging. Cellular senescence is a hallmark of aging and thought to be a tumor-suppressive mechanism with characteristic features such as irreversible growth arrest, apoptosis resistance, and an inflammatory senescence-associated secretory phenotype (SASP). Here, we review possible roles for cellular senescence in HO and how targeting senescent cells may provide new therapeutic approaches to both FOP and acquired forms of HO.

Aortic Valve Embryology, Mechanobiology, and Second Messenger Pathways: Implications for Clinical Practice

During the Renaissance, Leonardo Da Vinci was the first person to successfully detail the anatomy of the aortic root and its adjacent structures. Ever since, novel insights into morphology, function, and their interplay have accumulated, resulting in advanced knowledge on the complex functional characteristics of the aortic valve (AV) and root. This has shifted our vision from the AV as being a static structure towards that of a dynamic interconnected apparatus within the aortic root as a functional unit, exhibiting a complex interplay with adjacent structures via both humoral and mechanical stimuli. This paradigm shift has stimulated surgical treatment strategies of valvular disease that seek to recapitulate healthy AV function, whereby AV disease can no longer be seen as an isolated morphological pathology which needs to be replaced. As prostheses still cannot reproduce the complexity of human nature, treatment of diseased AVs, whether stenotic or insufficient, has tremendously evolved, with a similar shift towards treatments options that are more hemodynamically centered, such as the Ross procedure and valve-conserving surgery. Native AV and root components allow for an efficient Venturi effect over the valve to allow for optimal opening during the cardiac cycle, while also alleviating the left ventricle. Next to that, several receptors are present on native AV leaflets, enabling messenger pathways based on their interaction with blood and other shear-stress-related stimuli. Many of these physiological and hemodynamical processes are under-acknowledged but may hold important clues for innovative treatment strategies, or as potential novel targets for therapeutic agents that halt or reverse the process of valve degeneration. A structured overview of these pathways and their implications for cardiothoracic surgeons and cardiologists is lacking. As such, we provide an overview on embryology, hemodynamics, and messenger pathways of the healthy and diseased AV and its implications for clinical practice, by relating this knowledge to current treatment alternatives and clinical decision making.

Inflammation-associated ectopic mineralization

Ectopic mineralization refers to the deposition of mineralized complexes in the extracellular matrix of soft tissues. Calcific aortic valve disease, vascular calcification, gallstones, kidney stones, and abnormal mineralization in arthritis are common examples of ectopic mineralization. They are debilitating diseases and exhibit excess mortality, disability, and morbidity, which impose on patients with limited social or financial resources. Recent recognition that inflammation plays an important role in ectopic mineralization has attracted the attention of scientists from different research fields. In the present review, we summarize the origin of inflammation in ectopic mineralization and different channels whereby inflammation drives the initiation and progression of ectopic mineralization. The current knowledge of inflammatory milieu in pathological mineralization is reviewed, including how immune cells, pro-inflammatory mediators, and osteogenic signaling pathways induce the osteogenic transition of connective tissue cells, providing nucleating sites and assembly of aberrant minerals. Advances in the understanding of the underlying mechanisms involved in inflammatory-mediated ectopic mineralization enable novel strategies to be developed that may lead to the resolution of these enervating conditions.

Relation of Monocyte Number to Progression of Aortic Stenosis

Rapid progression of aortic stenosis (AS) is associated with poor prognosis. However, the relation between monocyte number and AS progression is unknown. Here, we detected the relation between monocyte number and AS progression. We retrospectively analyzed 220 patients with AS with at least 2 echocardiograms with the maximal interval ≥180 days from January 2016 to June 2021. AS severity was categorized by aortic jet velocity (V_max) and mean pressure gradient. Rapid progression of AS was defined when V_max increased ≥0.3 m/s/year. Patients were divided into low and high monocyte groups according to the cut-off value of the receiver-operating characteristic curve. AS progression was compared between the 2 groups. Various models of binary logistic regression were used to reveal the association between monocyte number and rapid progression. During a median of 601 days of echocardiographic follow-up (interquartile range 353 to 909), 52.7% of the population was in rapid progression. Patients in the high monocyte group had more rapid progression in both V_max and mean pressure gradient (p = 0.020 and p = 0.030, respectively). The percentage of patients with severe AS was increased by 5.4% in the low monocyte group and 16.9% in the high monocyte group. Different models of binary logistic regression showed that the monocyte number was positively associated with the rapid progression. In conclusion, a higher monocyte number was associated with the rapid progression of AS.

Development and validation of a new method to isolate, expand, and differentiate circulating osteogenic precursor (COP) cells

Circulating osteogenic precursor (COP) cells are a population of progenitor cells in the peripheral blood with the capacity to form bone in vitro and in vivo. They have characteristics of the mesenchymal stem and progenitor pool found in the bone marrow; however, more recently, a population of COP cells has been identified with markers of the hematopoietic lineage such as CD45 and CD34. While this population has been associated with several bone pathologies, a lack of cell culture models and inconsistent characterization has limited mechanistic research into their behavior and physiology. In this study, we describe a method for the isolation of CD45+/CD34+/alkaline phosphatase (ALP) + COP cells via fluorescence-activated cell sorting, as well as their expansion and differentiation in culture. Hematopoietic COP cells are a discreet population within the monocyte fraction of the peripheral blood mononuclear cells, which form proliferative, fibroblastoid colonies in culture. Their expression of hematopoietic markers decreases with time in culture, but they express markers of osteogenesis and deposit calcium with differentiation. It is hoped that this will provide a standard for their isolation, for consistency in future research efforts, to allow for the translation of COP cells into clinical settings.

The roles of immune cells in atherosclerotic calcification

Atherosclerosis is the leading cause of acute cardiovascular events, and vascular calcification is an important pathological phenomenon in atherosclerosis. Recently, many studies have shown that immune cells are closely associated with the development of atherosclerosis and calcification, but there are many conflicting viewpoints because of immune system complications, such as the pro-atherosclerotic and atheroprotective effects of regulatory B cells (Bregs), T helper type 2 (Th2) cells and T helper type 17 (Th17) cells. In this review, we summarize the studies on the roles of immune cells, especially lymphocytes and macrophages, in atherosclerotic calcification. Furthermore, we prepared graphs showing the relationship between T cells, B cells and macrophages and atherosclerotic calcification. Finally, we highlight some potential issues that are closely associated with the function of immune cells in atherosclerotic calcification. Based on current research results, this review summarizes the relationship between immune cells and atherosclerotic calcification, and it will be beneficial to understand the relationship of immune cells and atherosclerotic calcification.

Role of oxidative stress in calcific aortic valve disease and its therapeutic implications

Calcific aortic valve disease (CAVD) is the end result of active cellular processes that lead to the progressive fibrosis and calcification of aortic valve leaflets. In western populations, CAVD is a significant cause of cardiovascular morbidity and mortality, and in the absence of effective drugs, it will likely represent an increasing disease burden as populations age. As there are currently no pharmacological therapies available for preventing, treating, or slowing the development of CAVD, understanding the mechanisms underlying the initiation and progression of the disease is important for identifying novel therapeutic targets. Recent evidence has emerged of an important causative role for reactive oxygen species (ROS)-mediated oxidative stress in the pathophysiology of CAVD, inducing the differentiation of valve interstitial cells into myofibroblasts and then osteoblasts. In this review, we focus on the roles and sources of ROS driving CAVD and consider their potential as novel therapeutic targets for this debilitating condition.

Bone From Blood: Characteristics and Clinical Implications of Circulating Osteogenic Progenitor (COP) Cells

Circulating osteogenic progenitor (COP) cells are a population of cells in the peripheral blood with the capacity for bone formation, as well as broader differentiation into mesoderm-like cells in vitro. Although some of their biological characteristics are documented in vitro, their role in diseases of the musculoskeletal system remains yet to be fully evaluated. In this review, we provide an overview of the role of COP cells in a number of physiological and pathological conditions, as well as identify areas for future research. In addition, we suggest possible areas for clinical utilization in the management of musculoskeletal diseases. © 2020 American Society for Bone and Mineral Research (ASBMR).

Human aortic valve interstitial cells obtained from patients with aortic valve stenosis are vascular endothelial growth factor receptor 2 positive and contribute to ectopic calcification

Since aortic valve stenosis (AVS) is the most frequent and serious valvular heart disease in the elderly, and is accompanied by irreversible valve calcification, medicinal prevention of AVS is important. Although we recently demonstrated that human aortic valve interstitial cells (HAVICs) obtained from patients with AVS were highly sensitive to ectopic calcification stimulation, the cell types contributing to calcification are unknown. We aimed to immunocytochemically characterize HAVICs and identify their contribution to valve calcification. HAVICs were isolated from patients with AVS and cultured on non-coated dishes. Immunocytochemical features and HAVIC differentiation were analyzed in passage 1 (P1). The immunohistochemical features of the calcified aortic valve were analyzed. Most cultured P1 HAVICs were CD73-, CD90-, and CD105-positive, and CD45-and CD34-negative. HAVICs were vascular endothelial growth factor receptor 2 (VEGFR2)-positive; however, approximately half were α-smooth muscle actin (SMA)-positive, colonized, and easily differentiated into osteoblastic cells. Calcified aortic valve immunohistochemistry showed that all cells were positive for VEGFR2 and partly α-SMA. Further, VEGFR2-positive cells were more sensitive to tumor necrosis factor-α-induced ectopic calcification with or without α-SMA positivity. We conclude that HAVICs obtained from patients with AVS are VEGFR2-positive undifferentiated mesenchymal cells and may contribute to aortic valve ectopic calcification.

Interstitial cells in calcified aortic valves have reduced differentiation potential and stem cell-like properties

Valve interstitial cells (VICs) are crucial in the development of calcific aortic valve disease. The purpose of the present investigation was to compare the phenotype, differentiation potential and stem cell-like properties of cells from calcified and healthy aortic valves. VICs were isolated from human healthy and calcified aortic valves. Calcification was induced with osteogenic medium. Unlike VICs from healthy valves, VICs from calcified valves cultured without osteogenic medium stained positively for calcium deposits with Alizarin Red confirming their calcific phenotype. Stimulation of VICs from calcified valves with osteogenic medium increased calcification (p = 0.02), but not significantly different from healthy VICs. When stimulated with myofibroblastic medium, VICs from calcified valves had lower expression of myofibroblastic markers, measured by flow cytometry and RT-qPCR, compared to healthy VICs. Contraction of collagen gel (a measure of myofibroblastic activity) was attenuated in cells from calcified valves (p = 0.04). Moreover, VICs from calcified valves, unlike cells from healthy valves had lower potential to differentiate into adipogenic pathway and lower expression of stem cell-associated markers CD106 (p = 0.04) and aldehyde dehydrogenase (p = 0.04). In conclusion, VICs from calcified aortic have reduced multipotency compared to cells from healthy valves, which should be considered when investigating possible medical treatments of aortic valve calcification.

Fibroblasts and Their Pathological Functions in the Fibrosis of Aortic Valve Sclerosis and Atherosclerosis

Cardiovascular diseases, such as atherosclerosis and aortic valve sclerosis (AVS) are driven by inflammation induced by a variety of stimuli, including low-density lipoproteins (LDL), reactive oxygen species (ROS), infections, mechanical stress, and chemical insults. Fibrosis is the process of compensating for tissue injury caused by chronic inflammation. Fibrosis is initially beneficial and maintains extracellular homeostasis. However, in the case of AVS and atherosclerosis, persistently active resident fibroblasts, myofibroblasts, and smooth muscle cells (SMCs) perpetually remodel the extracellular matrix under the control of autocrine and paracrine signaling from the immune cells. Myofibroblasts also produce pro-fibrotic factors, such as transforming growth factor-β1 (TGF-β1), angiotensin II (Ang II), and interleukin-1 (IL-1), which allow them to assist in the activation and migration of resident immune cells. Post wound repair, these cells undergo apoptosis or become senescent; however, in the presence of unresolved inflammation and persistence signaling for myofibroblast activation, the tissue homeostasis is disturbed, leading to excessive extracellular matrix (ECM) secretion, disorganized ECM, and thickening of the affected tissue. Accumulating evidence suggests that diverse mechanisms drive fibrosis in cardiovascular pathologies, and it is crucial to understand the impact and contribution of the various mechanisms for the control of fibrosis before the onset of a severe pathological consequence.

Development of calcific aortic valve disease: Do we know enough for new clinical trials?

Calcific aortic valve disease (CAVD), previously thought to represent a passive degeneration of the valvular extracellular matrix (VECM), is now regarded as an intricate multistage disorder with sequential yet intertangled and interacting underlying processes. Endothelial dysfunction and injury, initiated by disturbed blood flow and metabolic disorders, lead to the deposition of low-density lipoprotein cholesterol in the VECM further provoking macrophage infiltration, oxidative stress, and release of pro-inflammatory cytokines. Such changes in the valvular homeostasis induce differentiation of normally quiescent valvular interstitial cells (VICs) into synthetically active myofibroblasts producing excessive quantities of the VECM and proteins responsible for its remodeling. As a result of constantly ongoing degradation and re-deposition, VECM becomes disorganised and rigid, additionally potentiating myofibroblastic differentiation of VICs and worsening adaptation of the valve to the blood flow. Moreover, disrupted and excessively vascularised VECM is susceptible to the dystrophic calcification caused by calcium and phosphate precipitating on damaged collagen fibers and concurrently accompanied by osteogenic differentiation of VICs. Being combined, passive calcification and biomineralisation synergistically induce ossification of the aortic valve ultimately resulting in its mechanical incompetence requiring surgical replacement. Unfortunately, multiple attempts have failed to find an efficient conservative treatment of CAVD; however, therapeutic regimens and clinical settings have also been far from the optimal. In this review, we focused on interactions and transitions between aforementioned mechanisms demarcating ascending stages of CAVD, suggesting a predisposing condition (bicuspid aortic valve) and drug combination (lipid-lowering drugs combined with angiotensin II antagonists and cytokine inhibitors) for the further testing in both preclinical and clinical trials.

Adaptive immune cells in calcific aortic valve disease

Calcific aortic valve disease (CAVD) is highly prevalent and has no pharmaceutical treatment. Surgical replacement of the aortic valve has proved effective in advanced disease but is costly, time limited, and in many cases not optimal for elderly patients. This has driven an increasing interest in noninvasive therapies for patients with CAVD. Adaptive immune cell signaling in the aortic valve has shown potential as a target for such a therapy. Up to 15% of cells in the healthy aortic valve are hematopoietic in origin, and these cells, which include macrophages, T lymphocytes, and B lymphocytes, are increased further in calcified specimens. Additionally, cytokine signaling has been shown to play a causative role in aortic valve calcification both in vitro and in vivo. This review summarizes the physiological presence of hematopoietic cells in the valve, innate and adaptive immune cell infiltration in disease states, and the cytokine signaling pathways that play a significant role in CAVD pathophysiology and may prove to be pharmaceutical targets for this disease in the near future.

Chronic inflammation: A key role in degeneration of bicuspid aortic valve

INTRODUCTION

Bicuspid aortic valve (BAV) is the most common congenital valvular heart defect resulting from abnormal aortic cusp formation during heart development, where two of the three normal and equal sized cusps fuse into a single large cusp resulting in a two cusps aortic valve. Over the past years, much interest has been given in understanding the pathogenesis of BAV and its complications. In this review, we focused on the role of inflammation, involved in the degeneration of BAV and the development of its complications.

ROLE OF INFLAMMATION

From a pathophysiological point of view, BAV may rapidly progress into aortic stenosis (AS) and is related to aortopathy. Several histopathologic studies have demonstrated that the development and progression of alterations in bicuspid aortic valve are related to an active process that includes: oxidative stress, shear stress, endothelial dysfunction, disorganized tissue architecture, inflammatory cells and cytokines. These factors are closely related one to each other, constituting the basis of the structural and functional alterations of the BAV.

CONCLUSION

Chronic inflammation plays a key role in the degeneration of BAV. Severe aortic stenosis in bicuspid aortic valves is associated with a more aggressive inflammatory process, increased inflammatory cells infiltration and neovascularization when compared to tricuspid AS. These findings might help to explain the more frequent onset and rapid progression of AS and the heavy aortic valve calcification seen in patients with BAV.

Heterotopic Ossification: A Comprehensive Review

Heterotopic ossification (HO) is a diverse pathologic process, defined as the formation of extraskeletal bone in muscle and soft tissues. HO can be conceptualized as a tissue repair process gone awry and is a common complication of trauma and surgery. This comprehensive review seeks to synthesize the clinical, pathoetiologic, and basic biologic features of HO, including nongenetic and genetic forms. First, the clinical features, radiographic appearance, histopathologic diagnosis, and current methods of treatment are discussed. Next, current concepts regarding the mechanistic bases for HO are discussed, including the putative cell types responsible for HO formation, the inflammatory milieu and other prerequisite “niche” factors for HO initiation and propagation, and currently available animal models for the study of HO of this common and potentially devastating condition. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Circulating osteogenic precursor cells: Building bone from blood

Circulating osteogenic precursor (COP) cells constitute a recently discovered population of circulating progenitor cells with the capacity to form not only bone but other mesenchymal tissues. There is a small, but growing body of literature exploring these cells, but with a great deal of disagreement and contradiction within it. This review explores the origins and biological characterization of these cells, including the identification strategies used to isolate these cells from the peripheral blood. It also examines the available knowledge on the in vitro and in vivo behaviour of these cells, in the areas of plastic adherence, differentiation capacity, proliferation, and cellular homing. We also review the implications for future use of COP cells in clinical practice, particularly in the area of regenerative medicine and the treatment and assessment of musculoskeletal disease.

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Circulating Osteogenic Precursors are circulating cells with characteristics of bone marrow mesenchymal stem cells.

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They are able to differentiate into bone, fat, cartilage and muscle, but many other characteristics remain unknown.

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They are heterogenous, with at least two specific populations present, with displaying different markers and behaviors.

Identification of circulating murine CD34+OCN+ cells

BACKGROUND AIMS

Previous studies identified a circulating human osteoblastic population that expressed osteocalcin (OCN), increased following fracture and pubertal growth, and formed mineralized colonies in vitro and bone in vivo. A subpopulation expressed CD34, a hematopoietic/endothelial marker. These findings led to our hypothesis that hematopoietic-derived CD34⁺OCN⁺ cells exist in the circulation of mice and are modulated after fracture.

METHODS

Flow cytometry was used to identify CD34⁺OCN⁺ cells in male B6.SJL-Ptprc^aPepc^b/BoyJ and Vav-Cre/mTmG (VavR) mice. Non-stabilized tibial fractures were created by three-point bend. Fractures were longitudinally imaged by micro-computed tomography, and immunofluorescent staining was used to evaluate CD34⁺OCN⁺ cells within fracture callus. AMD3100 (10 mg/kg) was injected subcutaneously for 3 days and the CD34⁺OCN⁺ population was evaluated by flow cytometry.

RESULTS

Circulating CD34⁺OCN⁺ cells were identified in mice and confirmed to be of hematopoietic origin (CD45⁺; Vav1⁺) using two mouse models. Both circulating and bone marrow-derived CD34⁺OCN⁺ cells peaked three weeks post-non-stabilized tibial fracture, suggesting association with cartilage callus transition to bone and early mineralization. Co-expression of CD34 and OCN in the fracture callus at two weeks post-fracture was observed. By three weeks, there was 2.1-fold increase in number of CD34⁺OCN⁺ cells, and these were observed throughout the fracture callus. AMD3100 altered CD34⁺OCN⁺ cell levels in peripheral blood and bone marrow.

DISCUSSION

Together, these data demonstrate a murine CD34⁺OCN⁺ circulating population that may be directly involved in fracture repair. Future studies will molecularly characterize CD34⁺OCN⁺ cells, determine mechanisms regulating their contribution, and examine if their number correlates with improved fracture healing outcomes.

Circulating osteogentic precursor cells in non-hereditary heterotopic ossification

Non-hereditary heterotopic ossification (NHHO) may occur after musculoskeletal trauma, central nervous system (CNS) injury, or surgery. We previously described circulating osteogenic precursor (COP) cells as a bone marrow-derived type 1 collagen⁺CD45⁺subpopulation of mononuclear adherent cells that are able of producing extraskeletal ossification in a murine in vivo implantation assay. In the current study, we performed a tissue analysis of COP cells in NHHO secondary to defined conditions, including traumatic brain injury, spinal cord injury, cerebrovascular accident, trauma without neurologic injury, and joint arthroplasty. All bone specimens revealed the presence of COP cells at 2-14 cells per high power field. COP cells were localized to early fibroproliferative and neovascular lesions of NHHO with evidence for their circulatory status supported by their presence near blood vessels in examined lesions. This study provides the first systematic evaluation of COP cells as a contributory histopathological finding associated with multiple forms of NHHO. These data support that circulating, hematopoietic-derived cells with osteogenic potential can seed inflammatory sites, such as those subject to soft tissue injury, and due to their migratory nature, may likely be involved in seeding sites distant to CNS injury.

Vascular ossification: Pathology, mechanisms, and clinical implications

In recent years, the mechanisms and clinical significance of vascular calcification have been increasingly investigated. For over a century, however, pathologists have recognized that vascular calcification is a form of heterotopic ossification. In this review, we aim to describe the pathology and molecular processes of vascular ossification, to characterize its clinical significance and treatment options, and to elucidate areas that require further investigation. The molecular mechanisms of vascular ossification involve the activation of regulators including bone morphogenic proteins and chondrogenic transcription factors and the loss of mineralization inhibitors like fetuin-A and pyrophosphate. Although few studies have examined the gross pathology of vascular ossification, the presence of these molecular regulators and evidence of microfractures and cartilage have been demonstrated on heart valves and atherosclerotic plaques. These changes are often triggered by common inflammatory and metabolic disorders like diabetes, hyperlipidemia, and chronic kidney disease. The increasing prevalence of these diseases warrants further research into the clinical significance of vascular ossification and future treatment options.

Conditional deletion of RB1 in the Tie2 lineage leads to aortic valve regurgitation

Objective

Aortic valve disease is a complex process characterized by valve interstitial cell activation, disruption of the extracellular matrix culminating in valve mineralization occurring over many years. We explored the function of the retinoblastoma protein (pRb) in aortic valve disease, given its critical role in mesenchymal cell differentiation including bone development and mineralization.

Approach and results

We generated a mouse model of conditional pRb knockout (cKO) in the aortic valve regulated by Tie2-Cre-mediated excision of floxed RB1 alleles. Aged pRb cKO animals showed significantly more aortic valve regurgitation by echocardiography compared to pRb het control animals. The pRb cKO aortic valves had increased leaflet thickness without increased cellular proliferation. Histologic studies demonstrated intense α-SMA expression in pRb cKO leaflets associated with disorganized extracellular matrix and increased leaflet stiffness. The pRb cKO mice also showed increased circulating cytokine levels.

Conclusions

Our studies demonstrate that pRb loss in the Tie2-lineage that includes aortic valve interstitial cells is sufficient to cause age-dependent aortic valve dysfunction.

Lamin A expression in circulating osteoprogenitors as a potential biomarker for frailty: The Nepean Osteoporosis and Frailty (NOF) Study

Lamin A is a protein of the nuclear lamina. Low levels of lamin A expression are associated with osteosarcopenia in mice. In this study, we hypothesized that low lamin A expression is also associated with frailty in humans. We aimed to develop a non-invasive method to quantify lamin A expression in epithelial and circulating osteoprogenitor (COP) cells, and to determine the relationship between lamin A expression and frailty in older individuals. COP cells and buccal swabs were obtained from 66 subjects (median age 74; 60% female; 26 non-frail, 23 pre-frail, and 17 frail) participating at the Nepean Osteoporosis and Frailty (NOF) Study. We quantified physical performance and disability, and stratified frailty in this population. Lamin A expression in epithelial and COP cells was quantified by flow cytometry. Linear regression models estimated the relationship between lamin A expression in buccal and COP cells, and prevalent disability and frailty. Lamin A expression in buccal cells showed no association with either disability or frailty. Low lamin A expression values in COP cells were associated with frailty. Frail individuals showed 60% lower levels of lamin A compared to non-frail (95% CI -36 to -74%, p<0.001) and 62% lower levels compared to pre-frail (95%CI -40 to -76%, p<0.001). In summary, we have identified lamin A expression in COP cells as a strong indicator of frailty. Further work is needed to understand lamin A expression as a risk stratifier, biomarker, or therapeutic target in frail older persons.

Age, gender, and percentage of circulating osteoprogenitor (COP) cells: The COP Study

Circulating osteoprogenitor (COP) cells are blood-borne cells which express a variety of osteoblastic markers and are able to form bone nodules in vivo. Whereas a high percentage of COP cells (%COP) is associated with vascular calcification, low %COP has been associated with disability and frailty. However, the reference range of %COP in age- and gender-matching populations, and the age-related changes in %COP remain unknown. A cross-sectional study was undertaken in 144 healthy volunteers in Western Sydney (20-90year-old, 10 male and 10 female subjects per decade). %COP was quantified by flow cytometry. A high inter-and intra-rater reliability was found. In average, in this healthy population average of %COP was 0.42. There was no significant difference in %COP among the age groups. Similarly, no significant difference was found in %COP with gender, weight, height or BMI. In addition, we identified a normal reference range of %COP of 0.1-3.8%. In conclusion, in addition to the identification of steady levels of COP cells with age, we also identified a normal reference range of %COP, which could be used in future studies looking at musculoskeletal diseases in older populations.

Circulating regulatory T cells in patients with aortic valve stenosis: Association with disease progression and aortic valve intervention

BACKGROUND

Severe aortic valve stenosis (AS) accounts for considerable morbidity and death, especially in older patients. There is increasing evidence to suggest a role for immune modulating cells in aortic valve (AV) degeneration. Regulatory T cells (Tregs) tune down inflammation. We aimed to study the levels of circulating Tregs in patients with AS and to assess their association with disease progression.

METHOD AND RESULTS

The number of Tregs (CD4+CD25+Foxp3+) was determined by flow cytometry in 229 patients with AS and a control group of 69 patients. Tregs were significantly higher in patients with AS compared to the control group (1.64± .61% vs 1.13±0.97%, p=0.04). In the logistic regression analysis, adjusted for baseline characteristics, only the hemoglobin level and Treg percent correlated with the presence of AS (OR 0.642 95% CI 0.512-0.805, p<0.001 and OR 1.411, 95% CI 1.080-1.844, p=0.011, respectively). One hundred patients underwent 2 echocardiographic studies during follow-up. The median decrease in AV area (AVA) was 0.1cm(2)/year. A borderline association was observed between Tregs and AVA progression (r=0.19, p=0.054). In a subgroup of 68 patients with severe AS, the association between Tregs and AVA progression was significant (r=0.374, p=0.0017). In addition, a drop in Treg levels was observed 3-6months after AV-intervention (1.86±1.6% vs 1.04±1.8%, p=0.0005).

CONCLUSIONS

Circulating Tregs are elevated in patients with AS. The levels of Tregs are higher in patients with severe AS and accelerated progression of valve narrowing. These results may help to identify AS patients with accelerated disease progression and possibly in need for earlier intervention.

Dysregulation of ossification-related miRNAs in circulating osteogenic progenitor cells obtained from patients with aortic stenosis

CAVD (calcific aortic valve disease) is the defining feature of AS (aortic stenosis). The present study aimed to determine whether expression of ossification-related miRNAs is related to differentiation intro COPCs (circulating osteogenic progenitor cells) in patients with CAVD. The present study included 46 patients with AS and 46 controls. Twenty-nine patients underwent surgical AVR (aortic valve replacement) and 17 underwent TAVI (transcatheter aortic valve implantation). The number of COPCs was higher in the AS group than in the controls (P<0.01). Levels of miR-30c were higher in the AS group than in the controls (P<0.01), whereas levels of miR-106a, miR-148a, miR-204, miR-211, miR-31 and miR-424 were lower in the AS group than in the controls (P<0.01). The number of COPCs and levels of osteocalcin protein in COPCs were positively correlated with levels of miR-30a and negatively correlated with levels of the remaining miRNAs (all P<0.05). The degree of aortic valve calcification was weakly positively correlated with the number of COPCs and miR-30c levels. The number of COPCs and miR-30c levels were decreased after surgery, whereas levels of the remaining miRNAs were increased (all P<0.05). Changes in these levels were greater after AVR than after TAVI (all P<0.05). In vitro study using cultured peripheral blood mononuclear cells transfected with each ossification-related miRNA showed that these miRNAs controlled levels of osteocalcin protein. In conclusion, dysregulation of ossification-related miRNAs may be related to the differentiation into COPCs and may play a significant role in the pathogenesis of CAVD.

Circulating CD14(+) monocytes in patients with aortic stenosis

BACKGROUND

Calcific aortic stenosis (AS) is an active process sharing similarities with atherosclerosis and chronic inflammation. The pathophysiology of AS is notable for three cardinal components: inflammation, fibrosis and calcification. Monocytes play a role in each of these processes. The role of circulating monocytes in AS is not clear. The aim of the present study was to study an association between circulating apoptotic and non apoptotic CD14(+) monocytes and AS features.

METHODS

We assessed the number of CD14(+) monocytes and apoptotic monocytes in 54 patients with significant AS (aortic valve area 0.74 ± 0.27 cm(2)) and compared them to 33 patients with similar risk factors and no valvular disease. The level of CD14(+) monocytes and apoptotic monocytes was assessed by flow cytometry.

RESULTS

There was no difference in the risk factor profile and known coronary or peripheral vascular diseases between patients with AS and controls. Patients with AS exhibited increased numbers of CD14(+) monocytes as compared to controls (9.9% ± 4.9% vs. 7.7% ± 3.9%, P = 0.03). CD14(+) monocyte number was related to age and the presence and severity of AS. In patients with AS, both CD14(+) monocytes and apoptotic monocytes were inversely related to aortic valve area.

CONCLUSIONS

Patients with significant AS have increased number of circulating CD14(+) monocytes and there is an inverse correlation between monocyte count and aortic valve area. These findings may suggest that inflammation is operative not only in early valve injury phase, but also at later developed stages such as calcification when AS is severe.

Pathophysiology of valvular heart disease

Valvular heart disease (VHD) is caused by either damage or defect in one of the four heart valves, aortic, mitral, tricuspid or pulmonary. Defects in these valves can be congenital or acquired. Age, gender, tobacco use, hypercholesterolemia, hypertension, and type II diabetes contribute to the risk of disease. VHD is an escalating health issue with a prevalence of 2.5% in the United States alone. Considering the likely increase of the aging population worldwide, the incidence of acquired VHD is expected to increase. Technological advances are instrumental in identifying congenital heart defects in infants, thereby adding to the growing VHD population. Almost one-third of elderly individuals have echocardiographic or radiological evidence of calcific aortic valve (CAV) sclerosis, an early and subclinical form of CAV disease (CAVD). Of individuals ages >60, ~2% suffer from disease progression to its most severe form, calcific aortic stenosis. Surgical intervention is therefore required in these patients as no effective pharmacotherapies exist. Valvular calcium load and valve biomineralization are orchestrated by the concerted action of diverse cell-dependent mechanisms. Signaling pathways important in skeletal morphogenesis are also involved in the regulation of cardiac valve morphogenesis, CAVD and the pathobiology of cardiovascular calcification. CAVD usually occurs without any obvious symptoms in early stages over a long period of time and symptoms are identified at advanced stages of the disease, leading to a high rate of mortality. Aortic valve replacement is the only primary treatment of choice. Biomarkers such as asymmetric dimethylarginine, fetuin-A, calcium phosphate product, natriuretic peptides and osteopontin have been useful in improving outcomes among various disease states. This review, highlights the current understanding of the biology of VHD, with particular reference to molecular and cellular aspects of its regulation. Current clinical questions and the development of new strategies to treat various forms of VHD medically were addressed.

The association of bone and osteoclasts with vascular calcification

The presence of bone tissue in calcified arteries may provide insights into the pathophysiology and potential reversibility of calcification, but the prevalence, distribution, and determinants of bone and osteoclasts in calcified arteries are unknown. Specimens of 386 arteries from lower limb amputations in 108 patients were examined retrospectively. Calcification was present in 282 arteries from 89 patients, which was medial in 64%, intimal in 9%, and both in 27%. Bone was present in 6% of arteries, essentially all of which were heavily calcified. Multiple sampling revealed that the true prevalence of bone in heavily calcified arteries was 25%. Bone was more common in medial rather than intimal calcifications (10% vs 3%, p=0.03) but did not vary with artery location (above vs below the knee). Heavily calcified arteries with bone were more likely to come from patients who were older (p=0.04), had diabetes (p=0.06), or were receiving warfarin (p=0.06), but there was no association with gender or renal failure. Bone was almost always adjacent to calcifications, along the periphery, but never within. Staining for the bone-specific proteins osteocalcin and osterix was noted in 20% and 45% of heavily calcified arteries without visible bone. Osteoclasts were present in 4.9% of arteries, all of which were heavily calcified and most of which contained bone. The frequent absence of bone in heavily calcified vessels and the histologic pattern strongly suggests a secondary rather than primary event. Recruitment of osteoclasts to vascular calcifications can occur but is rare, suggesting a limited capacity to reverse calcifications.

The pathology and pathobiology of bicuspid aortic valve: State of the art and novel research perspectives

Bicuspid aortic valve is the most prevalent cardiac valvular malformation. It is associated with a high rate of long‐term morbidity including development of calcific aortic valve disease, aortic regurgitation and concomitant thoracic aortic aneurysm and dissection. Recently, basic and translational studies have identified some key processes involved in the development of bicuspid aortic valve and its morbidity. The development of aortic valve disease and thoracic aortic aneurysm and dissection is the result of complex interactions between genotypes, environmental risk factors and specific haemodynamic conditions created by bicuspid aortic valve anatomy. Herein, we review the pathobiology of bicuspid aortic valve with a special emphasis on translational aspects of these basic findings. Important but unresolved problems in the pathology of bicuspid aortic valve and thoracic aortic aneurysm and dissection are discussed, along with the molecular processes involved.

The immunological contribution to heterotopic ossification disorders

The formation of bone outside the endogenous skeleton is a significant clinical event, rendering affected individuals with immobility and a diminished quality of life. This bone, termed heterotopic ossification (HO), can appear in patients following invasive surgeries and traumatic injuries, as well as progressively manifest in several congenital disorders. A unifying feature of both genetic and nongenetic episodes of HO is immune system involvement at the early stages of disease. Activation of the immune system sets the stage for the downstream anabolic events that eventually result in ectopic bone formation, rendering the immune system a particularly appealing site of early therapeutic intervention for optimal management of disease. In this review, we will discuss the immunological contributions to HO disorders, with specific focus on contributing cell types, signaling pathways, relevant in vivo animal models, and potential therapeutic targets.

In vitro models of aortic valve calcification: solidifying a system

Calcific aortic valve disease (CAVD) affects 25% of people over 65, and the late-stage stenotic state can only be treated with total valve replacement, requiring 85,000 surgeries annually in the US alone (University of Maryland Medical Center, 2013, http://umm.edu/programs/services/heart-center-programs/cardiothoracic-surgery/valve-surgery/facts). As CAVD is an age-related disease, many of the affected patients are unable to undergo the open-chest surgery that is its only current cure. This challenge motivates the elucidation of the mechanisms involved in calcification, with the eventual goal of alternative preventative and therapeutic strategies. There is no sufficient animal model of CAVD, so we turn to potential in vitro models. In general, in vitro models have the advantages of shortened experiment time and better control over multiple variables compared to in vivo models. As with all models, the hypothesis being tested dictates the most important characteristics of the in vivo physiology to recapitulate. Here, we collate the relevant pieces of designing and evaluating aortic valve calcification so that investigators can more effectively draw significant conclusions from their results.

Cardiac valve cells and their microenvironment--insights from in vitro studies

During every heartbeat, cardiac valves open and close coordinately to control the unidirectional flow of blood. In this dynamically challenging environment, resident valve cells actively maintain homeostasis, but the signalling between cells and their microenvironment is complex. When homeostasis is disrupted and the valve opening obstructed, haemodynamic profiles can be altered and lead to impaired cardiac function. Currently, late stages of cardiac valve diseases are treated surgically, because no drug therapies exist to reverse or halt disease progression. Consequently, investigators have sought to understand the molecular and cellular mechanisms of valvular diseases using in vitro cell culture systems and biomaterial scaffolds that can mimic the extracellular microenvironment. In this Review, we describe how signals in the extracellular matrix regulate valve cell function. We propose that the cellular context is a critical factor when studying the molecular basis of valvular diseases in vitro, and one should consider how the surrounding matrix might influence cell signalling and functional outcomes in the valve. Investigators need to build a systems-level understanding of the complex signalling network involved in valve regulation, to facilitate drug target identification and promote in situ or ex vivo heart valve regeneration.

CD34-negative mesenchymal stem-like cells may act as the cellular origin of human aortic valve calcification

Although various osteogenic inducers contribute to the calcification of human aortic valve interstitial cells, the cellular origin of calcification remains unclear. We immunohistochemically investigated the cellular origin of valve calcification using enzymatically isolated cells from both calcified and non-calcified human aortic valve specimens. CD73-, 90-, and 105-positive and CD45-negative mesenchymal stem-like cells (MSLCs) were isolated from both types of valve specimens using fluorescence-activated cell sorting. MSLCs were further sorted into CD34-negative and -positive cells. Compared with CD34-positive cells, CD34-negative MSLCs were significantly more sensitive to high inorganic phosphate (3.2 mM), calcifying easily in response. Furthermore, immunohistochemical staining showed that significantly higher numbers (~7-9-fold) of CD34-negative compared with CD34-positive MSLCs were localized in calcified aortic valve specimens obtained from calcified aortic stenosis patients. These results suggest that CD34-negative MSLCs are responsible for calcification of the aortic valve.

Molecular and cellular aspects of calcific aortic valve disease

Calcific aortic valve disease (CAVD) increasingly afflicts our aging population. One third of our elderly have echocardiographic or radiological evidence of calcific aortic valve sclerosis, an early and subclinical form of CAVD. Age, sex, tobacco use, hypercholesterolemia, hypertension, and type II diabetes mellitus all contribute to the risk of disease that has worldwide distribution. On progression to its most severe form, calcific aortic stenosis, CAVD becomes debilitating and devastating, and 2% of individuals >60 years are affected by calcific aortic stenosis to the extent that surgical intervention is required. No effective pharmacotherapies exist for treating those at risk for clinical progression. It is becoming increasingly apparent that a diverse spectrum of cellular and molecular mechanisms converge to regulate valvular calcium load; this is evidenced not only in histopathologic heterogeneity of CAVD, but also from the multiplicity of cell types that can participate in valve biomineralization. In this review, we highlight our current understanding of CAVD disease biology, emphasizing molecular and cellular aspects of its regulation. We end by pointing to important biological and clinical questions that must be answered to enable sophisticated disease staging and the development of new strategies to treat CAVD medically.

Myeloid calcifying cells promote atherosclerotic calcification via paracrine activity and allograft inflammatory factor-1 overexpression

Several cell types contribute to atherosclerotic calcification. Myeloid calcifying cells (MCCs) are monocytes expressing osteocalcin (OC) and bone alkaline phosphatase (BAP). Herein, we tested whether MCCs promote atherosclerotic calcification in vivo. We show that the murine spleen contains OC(+)BAP(+) cells with a phenotype similar to human MCCs, a high expression of adhesion molecules and CD11b, and capacity to calcify in vitro and in vivo. Injection of GFP(+) OC(+)BAP(+) cells into 8- or 40-week ApoE(-/-) mice led to more extensive calcifications in atherosclerotic areas after 24 or 4 weeks, respectively, compared to control OC(-)BAP(-) cells. Despite that OC(+)BAP(+) cells had a selective transendothelial migration capacity, tracking of the GFP signal revealed that presence of injected cells within atherosclerotic areas was an extremely rare event and so GFP mRNA was undetectable by qPCR of lesion extracts. By converse, injected OC(+)BAP(+) cells persisted in the bloodstream and bone marrow up to 24 weeks, suggesting a paracrine effect. Indeed, OC(+)BAP(+) cell-conditioned medium (CM) promoted calcification by cultured vascular smooth muscle cells (VSMC) more than CM from OC(-)BAP(-) cells. A genomic and proteomic investigation of MCCs identified allograft inflammatory factor (AIF)-1 as a potential candidate of this paracrine activity. AIF-1 stimulated VSMC calcification in vitro and monocyte-specific (CD11b-driven) AIF-1 overexpression in ApoE(-/-) mice increased calcium content in atherosclerotic areas. In conclusion, we show that murine OC(+)BAP(+) cells correspond to human MCCs and promote atherosclerotic calcification in ApoE(-/-) mice, through paracrine activity and modulation of resident cells by AIF-1 overexpression.

Role of circulating osteogenic progenitor cells in calcific aortic stenosis

OBJECTIVES

The purpose of this study was to determine the role of circulating endothelial progenitor cells with osteoblastic phenotype (EPC-OCN) in human aortic valve calcification (AVC).

BACKGROUND

Recent evidence suggests that rather than passive mineralization, AVC is an active atherosclerotic process with an osteoblastic component resembling coronary calcification. We have recently identified circulating EPCs with osteogenic properties carrying both endothelial progenitor (CD34, KDR) and osteoblastic (osteocalcin [OCN]) cell surface markers.

METHODS

Blood samples from controls (n = 22) and patients with mild to moderate calcific aortic stenosis (mi-moAS, n = 17), severe calcific AS (sAS, n = 26), and both sAS and severe coronary artery disease (sCAD) (n = 33) were collected during diagnostic coronary angiography. By using flow cytometry, peripheral blood mononuclear cells were analyzed for CD34, KDR, and OCN. Resected normal and calcified aortic valves were analyzed histologically.

RESULTS

Patients with mi-moAS and patients with sAS/sCAD had significantly less EPCs (CD34+/KDR+/OCN-) than controls. Patients with sAS showed significantly higher numbers of EPC-OCN (CD34+/KDR+/OCN+) than controls. In addition, the percentage of EPC costaining for OCN was higher in all disease groups compared with controls. A subgroup analysis of younger patients with bicuspid sAS showed a similar pattern of significantly lower EPCs but a high percentage of coexpression of OCN. Immunofluorescence showed colocalization of nuclear factor kappa-B and OCN in diseased and normal valves. CD34+/OCN+ cells were abundant in the endothelial and deeper cell layers of calcific aortic valve tissue but not in normal aortic valve tissue.

CONCLUSIONS

Circulating EPC-OCN may play a significant role in the pathogenesis and as markers of prognostication of calcific AS.

Circulating osteogenic precursor cells in type 2 diabetes mellitus

CONTEXT

Type 2 diabetes mellitus (T2D) is associated with an increased risk of fractures and low bone formation. However, the mechanism for the low bone formation is not well understood. Recently, circulating osteogenic precursor (COP) cells, which contribute to bone formation, have been characterized in the peripheral circulation.

OBJECTIVE

Our objective was to characterize the number and maturity of COP cells in T2D.

PATIENTS, DESIGN, AND SETTING

Eighteen postmenopausal women with T2D and 27 controls participated in this cross-sectional study at a clinical research center.

MAIN OUTCOME MEASURES

COP cells were characterized using flow cytometry and antibodies against osteocalcin (OCN) and early stem cell markers. Histomorphometric (n = 9) and molecular (n=14) indices of bone turnover and oxidative stress were also measured.

RESULTS

The percentage of OCN(+) cells in peripheral blood mononuclear cells was lower in T2D (0.8 ± 0.2 vs. 1.6 ± 0.4%; P < 0.0001), whereas the percentage of OCN(+) cells coexpressing the early marker CD146 was increased (OCN(+)/CD146(+): 33.3 ± 7 vs. 12.0 ± 4%; P < 0.0001). Reduced histomorphometric indices of bone formation were observed in T2D subjects, including mineralizing surface (2.65 ± 1.9 vs. 7.58 ± 2.4%, P = 0.02), bone formation rate (0.01 ± 0.1 vs. 0.05 ±0.2 μm(3)/um(2) · d, P = 0.02), and osteoblast surface (1.23 ±0.9 vs. 4.60 ± 2.5%, P = 0.03). T2D subjects also had reduced molecular expression of the osteoblast regulator gene Runx2 but increased expression of the oxidative stress markers p66(Shc) and SOD2.

CONCLUSIONS

Circulating OCN(+) cells were decreased in T2D, whereas OCN(+)/CD146(+) cells were increased. Histomorphometric indices of bone formation were decreased in T2D, as was molecular expression of osteoblastic activity. Stimulation of bone formation may have beneficial therapeutic skeletal consequences in T2D.

Arterial calcification and bone physiology: role of the bone-vascular axis

Bone never forms without vascular interactions. This simple statement of fact does not adequately reflect the physiological and pharmacological implications of the relationship. The vasculature is the conduit for nutrient exchange between bone and the rest of the body. The vasculature provides the sustentacular niche for development of osteoblast progenitors and is the conduit for egress of bone marrow cell products arising, in turn, from the osteoblast-dependent haematopoietic niche. Importantly, the second most calcified structure in humans after the skeleton is the vasculature. Once considered a passive process of dead and dying cells, vascular calcification has emerged as an actively regulated form of tissue biomineralization. Skeletal morphogens and osteochondrogenic transcription factors are expressed by cells within the vessel wall, which regulates the deposition of vascular calcium. Osteotropic hormones, including parathyroid hormone, regulate both vascular and skeletal mineralization. Cellular, endocrine and metabolic signals that flow bidirectionally between the vasculature and bone are necessary for both bone health and vascular health. Dysmetabolic states including diabetes mellitus, uraemia and hyperlipidaemia perturb the bone-vascular axis, giving rise to devastating vascular and skeletal disease. A detailed understanding of bone-vascular interactions is necessary to address the unmet clinical needs of an increasingly aged and dysmetabolic population.

Overactive bone morphogenetic protein signaling in heterotopic ossification and Duchenne muscular dystrophy

Bone morphogenetic proteins (BMPs) are important extracellular cytokines that play critical roles in embryogenesis and tissue homeostasis. BMPs signal via transmembrane type I and type II serine/threonine kinase receptors and intracellular Smad effector proteins. BMP signaling is precisely regulated and perturbation of BMP signaling is connected to multiple diseases, including musculoskeletal diseases. In this review, we will summarize the recent progress in elucidation of BMP signal transduction, how overactive BMP signaling is involved in the pathogenesis of heterotopic ossification and Duchenne muscular dystrophy, and discuss possible therapeutic strategies for treatment of these diseases.

Procalcific phenotypic drift of circulating progenitor cells in type 2 diabetes with coronary artery disease

Diabetes mellitus (DM) alters circulating progenitor cells relevant for the pathophysiology of coronary artery disease (CAD). While endothelial progenitor cells (EPCs) are reduced, there is no data on procalcific polarization of circulating progenitors, which may contribute to vascular calcification in these patients. In a cohort of 107 subjects with and without DM and CAD, we analyzed the pro-calcific versus endothelial differentiation status of circulating CD34+ progenitor cells. Endothelial commitment was determined by expression of VEGFR-2 (KDR) and pro-calcific polarization by expression of osteocalcin (OC) and bone alkaline phosphatase (BAP). We found that DM patients had significantly higher expression of OC and BAP on circulating CD34+ cells than control subjects, especially in the presence of CAD. In patients with DM and CAD, the ratio of OC/KDR, BAP/KDR, and OC+BAP/KDR was about 3-fold increased than in other groups. EPCs cultured from DM patients with CAD occasionally formed structures highly suggestive of calcified nodules, and the expression of osteogenic markers by EPCs from control subjects was significantly increased in response to the toll-like receptor agonist LPS. In conclusion, circulating progenitor cells of diabetic patients show a phenotypic drift toward a pro-calcific phenotype that may be driven by inflammatory signals.