Original ArticleAbundance and location of proteoglycans and hyaluronan within normal and myxomatous mitral valves
Introduction
Myxomatous mitral valve disease is a common valve abnormality with an incidence of 2.4–5% [1]. Myxomatous degeneration is the most common cause of mitral regurgitation in older patients [2], but the cause and progression of myxomatous disease remain unclear. Myxomatous mitral valves are characterized by floppy leaflets and elongated or ruptured chordae tendineae, which have profoundly weakened material behavior compared to normal valve tissues [3]. Previous histological studies of myxomatous valves have revealed collagen disruption [4], elastic fiber fragmentation [5], and accumulation of matrix metalloproteinases and glycosaminoglycans (GAGs) [6]. Currently, the only treatment for this disease is surgical repair; there are no medical therapies available to treat the matrix changes associated with myxomatous degeneration. Although there are a number of different theories regarding the pathogenesis of myxomatous mitral valve disease, including the diminished healing response of aging valve tissue to mechanical stress [2], genetic abnormalities [7], collagen and matrix dissolution [6], or abnormal accumulation of proteoglycans (PGs) [8], there is no consensus as to the mechanism of myxomatous degeneration. It is, however, evident by histological examination that GAGs and PGs, as a general matrix class, are abnormally distributed within myxomatous mitral valves, but exactly which PGs are overabundant within myxomatous valves has not been previously investigated.
Given the diverse nature of different PGs and their associated GAG chains, and their functions in mediating extracellular matrix (ECM) organization [9], it is compelling to ask which specific PG(s) could be involved in the remodeling and resultant dysfunction of myxomatous mitral valves. PGs consist of at least one GAG chain attached to a core protein. All GAG types exist as components of a PG, except for hyaluronan (HA), which is unsulfated and frequently presents as a free molecule, although HA may be noncovalently bound to the core protein of some of the larger hyalectin PGs through a link protein also known as hyaluronan-binding protein (HABP). The diverse group of PGs, as well as HA, performs many functions related to the growth, development, and pathology of tissues [10], and likely performs many such roles in heart valves as well. It has been shown that three particular PGs (decorin, biglycan, and versican) are present in the mitral valve and that their relative abundance varies according to the type of loading experienced by specific valve regions [11]. In the regions that experienced tension, the small leucine-rich PGs decorin and biglycan were more abundant; these PGs mediate collagen fibrillogenesis [12] and sequester transforming growth factor-β [13]. The large hyalectin PG versican was also found in the mitral valve; versican can aggregate with HA [14] to provide hydrated compressive resistance to tissues. Versican often colocalizes with elastic fibers and numerous cell-associated molecules to regulate cell adhesion, proliferation, and migration [14]. We have previously reported that there are elevated concentrations of selected GAG classes within myxomatous mitral valve chordae and posterior leaflets as compared to normal control tissues [15], but we have not investigated specific classes of PGs within these valves. Therefore, the primary purpose of this study was to assess the location and abundance of specific PGs and the GAG HA within myxomatous and normal mitral valves to improve our understanding of this common valve disease and to contribute towards developing novel treatments for this disease.
The secondary purpose of this study was an initial step towards investigating the mechanisms of turnover of PGs and GAGs, including HA, within heart valves. Understanding how the synthesis and degradation of PGs and GAGs are regulated within these tissues is important not only for myxomatous mitral valve disease but also for calcific aortic valve disease [16] and the design of bioprosthetic valves. In addition, understanding this regulation may lead to the development of novel medical therapies for valve disease or may offer opportunities for earlier intervention. Because gaining this knowledge will be a profound undertaking, in this particular study, we only investigated the potential for the turnover of HA in mitral valves via the hyaluronan receptor for endocytosis (HARE) [17]. HARE, also known as stabilin-2, is a cell receptor that enables the scavenging and clearance of HA and other GAGs from the blood [18]. Most abundant in lymph nodes, spleen, and liver, HARE has also been found in the heart valves of mice [19], but is absent in any other cardiovascular tissues. Based on the previously reported excess of HA in myxomatous tissues, it was speculated that the abundance of HARE in human myxomatous heart valves might be different from the abundance of HARE in control valves. Therefore, in this study, the abundance of specific PGs, HA, and HARE within normal and myxomatous mitral valves was measured using histochemical techniques.
Section snippets
Tissue procurement
Normal control mitral valve posterior leaflets were obtained after autopsy. Control subjects demonstrating any cardiovascular disease were excluded from this study. Myxomatous mitral valve posterior leaflets were surgically resected from patients undergoing surgical repair procedures to correct mitral regurgitation resulting from primary myxomatous degeneration (all had regurgitation Grade 3+ or 4+). The demographic makeup of all groups is shown in Table 1. Tissues for this study were provided
IHC for decorin, biglycan, and versican
Myxomatous valve sections showed overall higher PG staining intensity, with significantly greater abundance of decorin (P<.001), biglycan (P<.003), and versican (P<.03) as compared to normal valves (Fig. 1, Fig. 2). The staining intensity of versican in the myxomatous mitral valves was almost a full grade stronger than in the normal valves, and the staining intensities of decorin and biglycan were more than a full grade stronger in the myxomatous valves. The intensity of nonspecific staining in
Discussion
This study demonstrated differences in the abundance and localization of specific PGs within normal and myxomatous mitral valves. Decorin, biglycan, and versican, three PGs previously found in normal mitral valves [11], were present in significantly greater abundance in myxomatous valves. We did not find significant differences in the presence of HA within normal and myxomatous valve sections, although the abundance of HARE was stronger in normal valves. This is the first study to demonstrate
Summary
Because myxomatous mitral valves are widely reported to contain excess PGs, specific PGs and the GAG HA were histochemically localized within normal and myxomatous valves. The PGs decorin, biglycan, and versican were found in greater abundance in myxomatous valves compared to normal valves. Although HA abundance was equivalent, the HARE was less abundant in myxomatous valves.
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Cited by (0)
This research was funded by American Heart Association Scientist Development Grant 0235216N (K.J.G.-A.) and National Institutes of Health grants HL081558 (K.J.G.-A.) and GM69961 (P.H.W.).