Insight into pathologic abnormalities in congenital semilunar valve disease based on advances in understanding normal valve microstructure and extracellular matrix

Quantified planar collagen distribution in healthy and degenerative mitral valve: biomechanical and clinical implications

Degenerative mitral valve disease is a common valvular disease with two arguably distinct phenotypes: fibroelastic deficiency and Barlow's disease. These phenotypes significantly alter the microstructures of the leaflets, particularly the collagen fibers, which are the main mechanical load carriers. The predominant method of investigation is histological sections. However, the sections are cut transmurally and provide a lateral view of the microstructure of the leaflet, while the mechanics and function are determined by the planar arrangement of the collagen fibers. This study, for the first time, quantitatively examined planar collagen distribution quantitatively in health and disease using second harmonic generation microscopy throughout the thickness of the mitral valve leaflets. Twenty diseased samples from eighteen patients and six control samples were included in this study. Healthy tissue had highly aligned collagen fibers. In fibroelastic deficiency they are less aligned and in Barlow's disease they are completely dispersed. In both diseases, collagen fibers have two preferred orientations, which, in contrast to the almost constant one orientation in healthy tissues, also vary across the thickness. The results indicate altered in vivo mechanical stresses and strains on the mitral valve leaflets as a result of disease-related collagen remodeling, which in turn triggers further remodeling.

Dissecting Calcific Aortic Valve Disease-The Role, Etiology, and Drivers of Valvular Fibrosis

Calcific aortic valve disease (CAVD) is a highly prevalent and progressive disorder that ultimately causes gradual narrowing of the left ventricular outflow orifice with ensuing devastating hemodynamic effects on the heart. Calcific mineral accumulation is the hallmark pathology defining this process; however, fibrotic extracellular matrix (ECM) remodeling that leads to extensive deposition of fibrous connective tissue and distortion of the valvular microarchitecture similarly has major biomechanical and functional consequences for heart valve function. Significant advances have been made to unravel the complex mechanisms that govern these active, cell-mediated processes, yet the interplay between fibrosis and calcification and the individual contribution to progressive extracellular matrix stiffening require further clarification. Specifically, we discuss (1) the valvular biomechanics and layered ECM composition, (2) patterns in the cellular contribution, temporal onset, and risk factors for valvular fibrosis, (3) imaging valvular fibrosis, (4) biomechanical implications of valvular fibrosis, and (5) molecular mechanisms promoting fibrotic tissue remodeling and the possibility of reverse remodeling. This review explores our current understanding of the cellular and molecular drivers of fibrogenesis and the pathophysiological role of fibrosis in CAVD.

Biology and Biomechanics of the Heart Valve Extracellular Matrix

Heart valves are dynamic structures that, in the average human, open and close over 100,000 times per day, and 3 × 10⁹ times per lifetime to maintain unidirectional blood flow. Efficient, coordinated movement of the valve structures during the cardiac cycle is mediated by the intricate and sophisticated network of extracellular matrix (ECM) components that provide the necessary biomechanical properties to meet these mechanical demands. Organized in layers that accommodate passive functional movements of the valve leaflets, heart valve ECM is synthesized during embryonic development, and remodeled and maintained by resident cells throughout life. The failure of ECM organization compromises biomechanical function, and may lead to obstruction or leaking, which if left untreated can lead to heart failure. At present, effective treatment for heart valve dysfunction is limited and frequently ends with surgical repair or replacement, which comes with insuperable complications for many high-risk patients including aged and pediatric populations. Therefore, there is a critical need to fully appreciate the pathobiology of biomechanical valve failure in order to develop better, alternative therapies. To date, the majority of studies have focused on delineating valve disease mechanisms at the cellular level, namely the interstitial and endothelial lineages. However, less focus has been on the ECM, shown previously in other systems, to be a promising mechanism-inspired therapeutic target. Here, we highlight and review the biology and biomechanical contributions of key components of the heart valve ECM. Furthermore, we discuss how human diseases, including connective tissue disorders lead to aberrations in the abundance, organization and quality of these matrix proteins, resulting in instability of the valve infrastructure and gross functional impairment.

Animal Models for Heart Valve Research and Development

Valvular heart disease is the third-most common cause of heart problems in the United States. Malfunction of the valves can be acquired or congenital and each may lead either to stenosis or regurgitation, or even both in some cases. Heart valve disease is a progressive disease, which is irreversible and may be fatal if left untreated. Pharmacological agents cannot currently prevent valvular calcification or help repair damaged valves, as valve tissue is unable to regenerate spontaneously. Thus, heart valve replacement/repair is the only current available treatment. Heart valve research and development is currently focused on two parallel paths; first, research that aims to understand the underlying mechanisms for heart valve disease to emerge with an ultimate goal to devise medical treatment; and second, efforts to develop repair and replacement options for a diseased valve. Studies that focus on developmental malformation, genetic and disease epigenetics usually employ small animal models that are easy to access for in vivo imaging that minimally disturbs their environment during early stages of development. Alternatively, studies that aim to develop novel device for replacement and repair of diseased valves often employ large animals whose heart size and anatomy closely replicate human's. This paper aims to briefly review the current state-of-the-art animal models, and justification to use an animal model for a particular heart valve related project.

Hypoxia-mediated regulation of the secretory properties of mitral valve interstitial cells

The sophisticated function of the mitral valve depends to a large extent on its extracellular matrix (ECM) and specific cellular components. These are tightly regulated by a repertoire of mechanical stimuli and biological pathways. One potentially important stimulus is hypoxia. The purpose of this investigation is to determine the effect of hypoxia on the regulation of mitral valve interstitial cells (MVICs) with respect to the synthesis and secretion of extracellular matrix proteins. Hypoxia resulted in reduced production of total collagen and sulfated glycosaminoglycans (sGAG) in cultured porcine MVICs. Increased gene expression of matrix metalloproteinases-1 and -9 and their tissue inhibitors 1 and 2 was also observed after incubation under hypoxic conditions for up to 24 h. Hypoxia had no effect on MVIC viability, morphology, or phenotype. MVICs expressed hypoxia-inducible factor (HIF)-1α under hypoxia. Stimulating HIF-1α chemically caused a reduction in the amount of sGAG produced, similar to the effect observed under hypoxia. Human rheumatic valves had greater expression of HIF-1α compared with normal or myxomatous degenerated valves. In conclusion, hypoxia affects the production of certain ECM proteins and expression of matrix remodeling enzymes by MVICs. The effects of hypoxia appear to correlate with the induction of HIF-1α. This study highlights a potential role of hypoxia and HIF-1α in regulating the mitral valve, which could be important in health and disease.NEW & NOTEWORTHY This study demonstrates that hypoxia regulates extracellular matrix secretion and the remodeling potential of heart valve interstitial cells. Expression of hypoxia-induced factor-1α plays a role in these effects. These data highlight the potential role of hypoxia as a physiological mediator of the complex function of heart valve cells.

Heart valve health, disease, replacement, and repair: a 25-year cardiovascular pathology perspective

The past several decades have witnessed major advances in the understanding of the structure, function, and biology of native valves and the pathobiology and clinical management of valvular heart disease. These improvements have enabled earlier and more precise diagnosis, assessment of the proper timing of surgical and interventional procedures, improved prosthetic and biologic valve replacements and repairs, recognition of postoperative complications and their management, and the introduction of minimally invasive approaches that have enabled definitive and durable treatment for patients who were previously considered inoperable. This review summarizes the current state of our understanding of the mechanisms of heart valve health and disease arrived at through innovative research on the cell and molecular biology of valves, clinical and pathological features of the most frequent intrinsic structural diseases that affect the valves, and the status and pathological considerations in the technological advances in valvular surgery and interventions. The contributions of many cardiovascular pathologists and other scientists, engineers, and clinicians are emphasized, and potentially fruitful areas for research are highlighted.

A comparative anatomic and physiologic overview of the porcine heart

Despite advances during the last 2 decades in every aspect of cardiovascular research (interventional cardiology, cardiopulmonary resuscitation, and so forth), Western societies still are plagued by the consequences of cardiovascular disease. Consequently the discovery of new regimens and therapeutic interventions is of utmost importance. Research using human subjects is associated with substantial methodologic and ethical considerations, and the quest for an appropriate animal model for the human cardiovascular system has led to swine. The porcine heart bears a close resemblance to the human heart in terms of its coronary circulation and hemodynamic similarities and offers ease of implementation of methods and devices from human healthcare facilities. A thorough comprehension of the anatomy and physiology of the porcine cardiovascular system should focus on differences between swine and humans as well as similarities. Understanding these differences and similarities is essential to extrapolating data appropriately and to addressing the social demand for the ethical use of animals in biomedical research.

Glycosaminoglycan entrapment by fibrin in engineered heart valve tissues

Tissue engineered heart valves (TEHVs) may provide a permanent solution to congenital heart valve disease by permitting somatic valve growth in the pediatric patient. However, to date, TEHV studies have focused primarily on collagen, the dominant component of valve extracellular matrix (ECM). Temporal decreases in other ECM components, such as the glycosaminoglycans (GAGs), generally decrease as cells produce more collagen under mechanically loaded states; nevertheless, GAGs represent a key component of the valve ECM, providing structural stability and hydration to the leaflets. In an effort to retain GAGs within the engineered constructs, here we investigated the utility of the protein fibrin in combination with a valve-like, cyclic flexure and steady flow (flex-flow) mechanical conditioning culture process using adult human periodontal ligament cells (PLCs). We found both fibrin and flex-flow mechanical components to be independently significant (p<0.05), and hence important in influencing the DNA, GAG and collagen contents of the engineered tissues. In addition, the interaction of fibrin with flex-flow was found to be significant in the case of collagen; specifically, the combination of these environments promoted PLC collagen production resulting in a significant difference compared to dynamic and statically cultured specimens without fibrin. Histological examination revealed that the GAGs were retained by fibrin entrapment and adhesion, which were subsequently confirmed by additional experiments on native valve tissues. We conclude that fibrin in the flex-flow culture of engineered heart valve tissues: (i) augments PLC-derived collagen production; and (ii) enhances retention of GAGs within the developing ECM.

Insufficient versican cleavage and Smad2 phosphorylation results in bicuspid aortic and pulmonary valves

Bicuspid or bifoliate aortic valve (BAV) results in two rather than three cusps and occurs in 1-2% of the population placing them at higher risk of developing progressive aortic valve disease. Only NOTCH-1 has been linked to human BAV, and genetically modified mouse models of BAV are limited by low penetrance and additional malformations. Here we report that in the Adamts5(-/-) valves, collagen I, collagen III, and elastin were disrupted in the malformed hinge region that anchors the mature semilunar cusps and where the ADAMTS5 proteoglycan substrate versican, accumulates. ADAMTS5 deficient prevalvular mesenchyme also exhibited a reduction of α-smooth muscle actin and filamin A suggesting versican cleavage may be involved in TGFβ signaling. Subsequent evaluation showed a significant decrease of pSmad2 in regions of prevalvular mesenchyme in Adamts5(-/-) valves. To test the hypothesis that ADAMTS5 versican cleavage is required, in part, to elicit Smad2 phosphorylation we further reduced Smad2 in Adamts5(-/-) mice through intergenetic cross. The Adamts5(-/-);Smad2(+/-) mice had highly penetrant BAV and bicuspid pulmonary valve (BPV) malformations as well as increased cusp and hinge size compared to the Adamts5(-/-) and control littermates. These studies demonstrate that semilunar cusp malformations (BAV and BPV) can arise from a failure to remodel the proteoglycan-rich provisional ECM. Specifically, faulty versican clearance due to ADAMTS5 deficiency blocks the initiation of pSmad2 signaling, which is required for excavation of endocardial cushions during aortic and pulmonary valve development. Further studies using the Adamts5(-/-); Smad2(+/-) mice with highly penetrant and isolated BAV, may lead to new pharmacological treatments for valve disease.

Endothelial deletion of ADAM17 in mice results in defective remodeling of the semilunar valves and cardiac dysfunction in adults

Global inactivation of the metalloproteinase ADAM17 during mouse development results in perinatal lethality and abnormalities of the heart, including late embryonic cardiomegaly and thickened semilunar and atrioventricular valves. These defects have been attributed in part to a lack of ADAM17-mediated processing of HB-EGF, as absence of soluble HB-EGF results in similar phenotypes. Because valvular mesenchymal cells are largely derived from cardiac endothelial cells, we generated mice with a floxed Adam17 allele and crossed these animals with Tie2-Cre transgenics to focus on the role of endothelial ADAM17 in valvulogenesis. We find that although hearts from late-stage embryos with ablation of endothelial ADAM17 appear normal, an increase in valve size and cell number is evident, but only in the semilunar cusps. Unlike Hbegf(-/-) valves, ADAM17-null semilunar valves do not differ from controls in acute cell proliferation at embryonic day 14.5 (E14.5), suggesting compensatory processing of HB-EGF. However, levels of the proteoglycan versican are significantly reduced in mutant hearts early in valve remodeling (E12.5). After birth, aortic valve cusps from mutants are not only hyperplastic but also show expansion of the glycosaminoglycan-rich component, with the majority of adults exhibiting aberrant compartmentalization of versican and increased deposition of collagen. The inability of mutant outflow valve precursors to transition into fully mature cusps is associated with decreased postnatal viability, progressive cardiomegaly, and systolic dysfunction. Together, our data indicate that ADAM17 is required in valvular endothelial cells for regulating cell content as well as extracellular matrix composition and organization in semilunar valve remodeling and homeostasis.