Elsevier

Cardiovascular Pathology

Volume 18, Issue 4, July–August 2009, Pages 191-197
Cardiovascular Pathology

Original Article
Abundance and location of proteoglycans and hyaluronan within normal and myxomatous mitral valves

https://doi.org/10.1016/j.carpath.2008.05.001Get rights and content

Abstract

Introduction

Extracellular matrix changes occur in many heart valve pathologies. For example, myxomatous mitral valves are reported to contain excess proteoglycans and hyaluronan. However, it is unknown which specific proteoglycans are altered in myxomatous valves. Because proteoglycans perform varied functions in connective tissues, this study was designed to identify and localize three matrix-associated proteoglycans, as well as hyaluronan and the hyaluronan receptor for endocytosis, within myxomatous and normal mitral valves.

Methods

Human mitral posterior leaflets (control, n=6–9; myxomatous, n=14–21; mean age, 61 years for all groups) were histochemically stained for proteoglycan core proteins, hyaluronan, and the hyaluronan receptor for endocytosis. Stain intensity was semiquantitatively graded to determine differences in marker abundance between normal and myxomatous valves. The proteoglycans were localized to different regions of the leaflet by correspondence to parallel Movat-stained sections.

Results

The proteoglycans decorin, biglycan, and versican were more abundant in myxomatous valves than in normal controls (P<.03). There was a gender effect on proteoglycan presence, but no age-related trends were observed. Hyaluronan and the hyaluronan receptor for endocytosis were distributed throughout all valves. There was no significant difference in hyaluronan between groups, but expression of the hyaluronan receptor for endocytosis was reduced in myxomatous valves compared to normal controls (P<.002).

Conclusion

Excess decorin, biglycan, and versican may be associated with the remodeling of other matrix components in myxomatous mitral valves. Decreased expression of the hyaluronan receptor for endocytosis in myxomatous valves suggests that hyaluronan metabolism could be altered in myxomatous mitral valve disease. These findings contribute towards elucidating the pathogenesis of myxomatous mitral valve disease and developing potential new therapies.

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.).

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