**1. Introduction**

Calcific Aortic Valve Disease (CAVD) occurs in >2% of the population over 65 years of age and often leads to valvular stenosis that necessitates valve replacement [1]. CAVD is a pro‐ gressive disease, often manifesting first as aortic valve sclerosis and later developing into stenosis and valve dysfunction [2]. The specific molecular and cellular mechanisms of CAVD initiation and advancement are not well defined, and inhibitors of CAVD progres‐ sion have not been identified. The current standard of treatment for CAVD is aortic valve replacement [3]. Presently, there are no pharmacologic-based treatments for CAVD, and new therapeutic approaches for CAVD are needed. The majority of aortic valves that are re‐ placed have congenital malformations, such as bicuspid aortic valve (BAV), establishing a link between valve development and disease mechanisms [4].

The molecular mechanisms of CAVD include activation of signaling pathways implicated in both heart valve development (valvulogenesis) and bone development (osteogenesis) [5-8]. These include activation of regulators of progenitor specification, cell proliferation, and dif‐ ferentiation. Heart valves and bone are complex connective tissues with compartmentalized ECM produced by specialized cell types. Over the past several years, extensive progress has been made in defining molecular regulatory mechanisms in heart valve and bone develop‐ ment (Reviewed in [8-10]). Strikingly, regulatory pathways that control development of car‐ tilage, tendon and bone also are active in developing valves [8, 11]. In addition, recent studies have reported induction of molecular regulators of valvulogenesis and osteogenesis in CAVD [7, 12-14]. However, it is not known if these developmental mechanisms have rep‐ arative functions or contribute to the progression of CAVD.

Here we review molecular mechanisms of valve and bone development as they relate to mo‐ lecular mechanisms of CAVD. Recent studies have provided evidence for the involvement of specific regulatory pathways in CAVD as activators or inhibitors of disease progression.

© 2013 Wirrig and Yutzey; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Additional research in animal models and human patient specimens is necessary to deter‐ mine the detrimental molecular regulatory pathways that promote CAVD progression and also beneficial pathways that potentially inhibit CAVD. In the future, manipulation of these pathways could be exploited therapeutically in the treatment of patients with CAVD or with aortic valve sclerosis that precedes calcification.

luronan (Figure 1) [15]. These swellings are invested with mesenchymal cells that arise from endothelial-to-mesenchymal transformation (EMT) of the endocardium [16]. Similar swel‐ ling and induction of EMT occur approximately a day later in the cardiac OFT cushions that will contribute to the semilunar valves [17]. Endocardial EMT is induced by signaling mole‐ cules, including bone morphogenetic proteins (BMPs), emanating from the adjacent myocar‐ dium in the AVC and OFT [8, 18-20]. Once established, the endocardial cushions expand through increased extracellular matrix (ECM) production and cell proliferation of mesen‐ chymal and endothelial cells. The AV cushions subsequently fuse to separate right and left cardiac channels. In addition, lateral cushions are induced in the AV sulcus that will give rise to the mural leaflets of the mitral and tricuspid valves [21]. Neural crest cells (NCCs) migrate into the cushions of the cardiac OFT, contributing to the septum between the aortic and pulmonic roots and also to the morphogenesis of individual semilunar valve leaflets [21, 22]. At this point, distinct primordia of individual valve leaflets become apparent and proliferation of valve interstitial cells (VICs) is reduced [23]. Valve morphogenesis occurs with elongation and thinning of the valve primordia, in addition to ECM remodeling and stratification. In general, the development of the AV and semilunar (SL) valves is similar, but there are some differences in the sources of cells and structure of the resulting leaflets [8, 10, 11, 24]. In mature SL and AV valves, the ECM is stratified into collagen-rich fibrosa, pro‐ teoglycan-rich spongiosa, and elastin-rich (atrialis-ventricularis) layers oriented relative to

Developmental Pathways in CAVD http://dx.doi.org/10.5772/54356 61

The primary embryonic source of adult semilunar valve interstitial cells is the endothelialderived cells of the endocardial cushions, that arise as a result of EMT as determined by Tie2-Cre lineage tracing in mice [23, 25]. Since the cardiac OFT is derived from the secon‐ dary heart field (SHF), semilunar VICs derived from OFT endocardium also are SHF-de‐ rived [20, 26]. NCC-derived cells are present in adult mouse semilunar valve leaflets as demonstrated by cell lineage tracing with Wnt1-Cre [27]. These cells are predominant throughout the aortic and pulmonic valve leaflets, but are enriched in the leaflets adjacent to the aorticopulmonary septum, which also is derived from NCCs [21, 28]. NCCs are required for semilunar valve morphogenesis and remodeling, likely by providing signals necessary for cell lineage differentiation and leaflet maturation [29, 30]. Another potential source of VICs is the epicardium, which contributes cells to the parietal leaflets of AV valves [31]. However, epicardial-derived cells (EPDCs) have not been not reported to contribute to the semilunar valves, based on Wt1-Cre fatemapping studies [31, 32]. Recent studies have re‐ ported that bone marrow-derived stem cells (BMSCs) are present in the developing and ma‐ ture semilunar valves [33, 34]. Additional work is necessary to determine if these cells have lineages and functions distinct from the predominant endocardial cushion-derived or neural crest-derived VICs. It is possible that valve cell lineages derived from different developmen‐ tal sources have distinct functions in normal and diseased aortic valves, but this has not yet been demonstrated. The sources of increased proliferative cells in diseased valves are rela‐ tively unknown, but could be any of these embryonic sources or, alternatively, an infiltrat‐

blood flow [24].

ing cell type.

**2.2. Embryonic origins of valve cell lineages**

**Figure 1. Molecular pathways active during endocardial cushion development and valve stratification are reacti‐ vated in CAVD.** (A) Early stages of OFT cushion development are marked by ECM deposition, EMT, and neural crest cell in‐ filtration. Factors necessary for EMT and mesenchymal cell function are expressed. (B) During late embryonic development and early postnatal development, the aortic valve becomes stratified and possesses three ECM layers. Factors necessary for ECM remodeling are active at this stage. (C) In CAVD, the ECM remodels and the valve becomes thickened. Calcification (black nodules) is typically observed in the collagen-rich fibrosa layer. Many factors expressed during OFT cushion develop‐ ment and valve stratification are reactivated during disease. Furthermore, osteogenic factors involved in bone develop‐ ment are also observed in CAVD. Please see text for details and references. OFT = outflow tract, EMT = epithelial-tomesenchymal transition, ECM = extracellular matrix, CAVD = calcific aortic valve disease.
