Fibrocalcific aortic valve disease: opportunity to understand disease mechanisms using mouse models

Abstract

Studies in vitro and in vivo continue to identify complex-regulated mechanisms leading to overt fibrocalcific aortic valve disease (FCAVD). Assessment of the functional impact of those processes requires careful studies of models of FCAVD in vivo. Although the genetic basis for FCAVD is unknown for most patients with FCAVD, several disease-associated genes have been identified in humans and mice. Some gene products which regulate valve development in utero also protect against fibrocalcific disease during postnatal aging. Valve calcification can occur via processes that resemble bone formation. But valve calcification can also occur by nonosteogenic mechanisms, such as formation of calcific apoptotic nodules. Anticalcific interventions might preferentially target either osteogenic or nonosteogenic calcification. Although FCAVD and atherosclerosis share several risk factors and mechanisms, there are fundamental differences between arteries and the aortic valve, with respect to disease mechanisms and responses to therapeutic interventions. Both innate and acquired immunity are likely to contribute to FCAVD. Angiogenesis is a feature of inflammation, but may also contribute independently to progression of FCAVD, possibly by actions of pericytes that are associated with new blood vessels. Several therapeutic interventions seem to be effective in attenuating the development of FCAVD in mice. Therapies which are effective early in the course of FCAVD, however, are not necessarily effective in established disease.

Keywords: aortic valve; aortic valve calcification; aortic valve stenosis.

Figure 1. Overestimation of aortic stenosis severity in the presence of aortic regurgitation

A: Color Doppler from an eNOS^−/− mouse demonstrates moderate-to-severe aortic regurgitation (arrows). B: LV-to-aorta pressure gradient = 55 mmHg in the same mouse. C: M-mode echocardiogram demonstrates normal aortic cusp separation = 1.2 mm (vertical white bar), i.e. absence of aortic stenosis, in the same mouse. D: Relationship between aortic valve systolic cusp separation and continuous-wave Doppler transvalvular blood velocity in the presence (N = 86) or absence (N = 699) of aortic regurgitation in fat-fed Ldlr^−/−Apob100/100/Mttp^fl/flMx1Cre^+/+ mice, ages 3 – 12 months. Arrow indicates data from mice that have aortic regurgitation, but mild or absent aortic stenosis, with significantly elevated transvalvular velocities. In all cases, the angle of exit between the Doppler line and the systolic blood flow vector was ≤ 60°.

Figure 2. Overestimation of stenosis severity

A. Calculation of transvalvular blood flow velocity requires angle correction when the direction of flow is not colinear with the line of Doppler interrogation. In the illustration, the angle of exit of presumed blood flow is 75° . Thus, measured velocity would be divided by the cosine of 75°, = 0.26, in order to compute actual flow velocity. When transvalvular blood flow is disturbed, a portion of the flow jet will be directed more parallel to the Doppler line, and will be detected as a higher velocity. In this case, correction for a presumed angle of exit of 75° will overestimate true blood velocity by a factor of about 4. B. Doppler velocimetry from a normal mouse, with neither aortic stenosis nor regurgitation. The angle between the Doppler cursor and the presumed direction of transvalvular flow was 75° (image not shown). Peak velocity is reported as = 3.6 m/s, erroneously predicting a peak valve gradient of 52 mmHg. C. M-mode echocardiogram of the aortic valve in the same mouse. White bar indicates aortic cusp separation = 1.3 mm, which is normal. Color Doppler and magnetic resonance imaging both confirmed the absence of aortic regurgitation (images not shown).

Figure 3. Scanning electron micrographs of trileaflet (left) and bicuspid (right) aortic valves

The eccentric structure of valve cusps and their attachments to the aortic annulus present challenges for quantitation of mechanical stresses or severity of stenosis. Left: normal trileaflet aortic valve at 12 months of age. Right: congenitally bicuspid aortic valve from an eNOS^−/− mouse, at 6 months of age, with cusp thickening (*) and cusp fusion (CF).

Figure 4. Interactions between genotype and environmental factors in the development of FCAVD

Mutations which interfere with epithelial-to-mesenchymal transformation (EMT) predispose to formation of a congenitally bicuspid valve and, in some cases, predispose to calcific aortic stenosis. Knockdown of Alk2 perturbs valve development following EMT and predisposes to development of a functionally bicuspid aortic valve, but does not produce overt valve calcification. Mild aortic valve sclerosis is a predictable consequence of normal aging. But in the presence of valve injury, or disease-prone genes, congenitally normal valves can develop overt FCAVD. Opn osteopontin. See text for additional abbreviations.

Figure 5. Speculative and simplified diagram of disease pathways in cusps of the aortic valve

Left: Valvular interstitial cells (VICs), when activated, transdifferentiate to myofibroblasts or osteoblasts, which produce collagen and calcification. Pericytes, near new blood vessels in the valve, also may transdifferentiate to myofibroblasts. Vascular endothelial growth factor-A (VEGF-A) may stimulate differentiation of myofibroblasts to osteoblasts. Right: When mature dendritic cells, primarily at the base of the valve, bind to naïve T-cells, the T-cells are activated. Interleukin(IL)-1, IL-17, interferon (IFN)-γ and other proinflammatory cytokines produce inflammation and may contribute to calcification. TNF-α, released by monocyte/macrophages and activated T-cells, also plays a key role in inflammation and calcification in the valve. (illustration credit: Ben Smith).