Inhibition of calcifying nodule formation in cultured porcine aortic valve cells by nitric oxide donors

The congenital bicuspid aortic valve can experience high-frequency unsteady shear stresses on its leaflet surface

The bicuspid aortic valve (BAV) is a common congenital malformation of the aortic valve (AV) affecting 1% to 2% of the population. The BAV is predisposed to early degenerative calcification of valve leaflets, and BAV patients constitute 50% of AV stenosis patients. Although evidence shows that genetic defects can play a role in calcification of the BAV leaflets, we hypothesize that drastic changes in the mechanical environment of the BAV elicit pathological responses from the valve and might be concurrently responsible for early calcification. An in vitro model of the BAV was constructed by surgically manipulating a native trileaflet porcine AV. The BAV valve model and a trileaflet AV (TAV) model were tested in an in vitro pulsatile flow loop mimicking physiological hemodynamics. Laser Doppler velocimetry was used to make measurements of fluid shear stresses on the leaflet of the valve models using previously established methodologies. Furthermore, particle image velocimetry was used to visualize the flow fields downstream of the valves and in the sinuses. In the BAV model, flow near the leaflets and fluid shear stresses on the leaflets were much more unsteady than for the TAV model, most likely due to the moderate stenosis in the BAV and the skewed forward flow jet that collided with the aorta wall. This additional unsteadiness occurred during mid- to late-systole and was composed of cycle-to-cycle magnitude variability as well as high-frequency fluctuations about the mean shear stress. It has been demonstrated that the BAV geometry can lead to unsteady shear stresses under physiological flow and pressure conditions. Such altered shear stresses could play a role in accelerated calcification in BAVs.

Calcific nodule morphogenesis by heart valve interstitial cells is strain dependent

Calcific aortic valve disease (CAVD) results in impaired function through the inability of valves to fully open and close, but the causes of this pathology are unknown. Stiffening of the aorta is associated with CAVD and results in exposing the aortic valves to greater mechanical strain. Transforming growth factor β1 (TGF-β1) is enriched in diseased valves and has been shown to combine with strain to synergistically alter aortic valve interstitial cell (AVIC) phenotypes. Therefore, we investigated the role of strain and TGF-β1 on the calcification of AVICs. Following TGF-β1 pretreatment, strain induced intact monolayers to aggregate and calcify. Using a wound assay, we confirmed that TGF-β1 increases tension in the monolayer in parallel with α-smooth muscle actin (αSMA) expression. Continual exposure to strain accelerates aggregates to calcify into mature nodules that contain a necrotic core surrounded by an apoptotic ring. This phenotype appears to be mediated by strain inhibition of AVIC migration after the initial formation of aggregates. To better interpret the extent to which externally applied strain physically impacts this process, we modified the classical Lamé solution, derived using principles from linear elasticity, to reveal strain magnification as a novel feature occurring in a mechanical environment that supports nodule formation. These results indicate that strain can impact multiple points of nodule formation: by modifying tension in the monolayer, remodeling cell contacts, migration, apoptosis, and mineralization. Therefore, strain-induced nodule formation provides new directions for developing strategies to address CAVD.

The antioxidant tempol ameliorates arterial medial calcification in uremic rats: important role of oxidative stress in the pathogenesis of vascular calcification in chronic kidney disease

Vascular calcification is closely related to cardiovascular morbidity and mortality. Accumulating data indicate that oxidative stress is associated with dysfunction of various organs, including cardiovascular diseases in chronic kidney disease (CKD). However, it remains undetermined if oxidative stress induced by uremia promotes arterial medial calcification. The present study investigated the role of oxidative stress in the pathogenesis of arterial medial calcification in uremic rats. Rats with uremia induced by adenine-rich diet progressively developed arterial medial calcification, which was accompanied by time-dependent increases in both aortic and systemic oxidative stress. Immunohistochemical and biochemical analyses showed that the arterial medial calcification progressed in a time-dependent manner that is parallel to the osteogenic transdifferentiation of vascular smooth muscle cells. Accumulation of oxidative stress was also identified in the calcified regions. Time-course studies indicated that both oxidative stress and hyperphosphatemia correlated with arterial medial calcification. Tempol, an antioxidant, ameliorated osteogenic transdifferentiation of vascular smooth muscle cells and arterial medial calcification in uremic rats, together with reduction in aortic and systemic oxidative stress levels, without affecting serum biochemical parameters. Our data suggest that oxidative stress induced by uremia can play a role in the pathogenesis of vascular calcification in CKD, and that antioxidants such as tempol are potentially useful in preventing the progression of vascular calcification in CKD.

Calcific aortic valve stenosis: methods, models, and mechanisms

Calcific aortic valve stenosis (CAVS) is a major health problem facing aging societies. The identification of osteoblast-like and osteoclast-like cells in human tissue has led to a major paradigm shift in the field. CAVS was thought to be a passive, degenerative process, whereas now the progression of calcification in CAVS is considered to be actively regulated. Mechanistic studies examining the contributions of true ectopic osteogenesis, nonosseous calcification, and ectopic osteoblast-like cells (that appear to function differently from skeletal osteoblasts) to valvular dysfunction have been facilitated by the development of mouse models of CAVS. Recent studies also suggest that valvular fibrosis, as well as calcification, may play an important role in restricting cusp movement, and CAVS may be more appropriately viewed as a fibrocalcific disease. High-resolution echocardiography and magnetic resonance imaging have emerged as useful tools for testing the efficacy of pharmacological and genetic interventions in vivo. Key studies in humans and animals are reviewed that have shaped current paradigms in the field of CAVS, and suggest promising future areas for research.

Ramipril retards development of aortic valve stenosis in a rabbit model: mechanistic considerations

BACKGROUND AND PURPOSE

Aortic valve stenosis (AVS) is associated with significant cardiovascular morbidity and mortality. To date, no therapeutic modality has been shown to be effective in retarding AVS progression. We evaluated the effect of angiotensin-converting enzyme inhibition with ramipril on disease progression in a recently developed rabbit model of AVS.

EXPERIMENTAL APPROACH

The effects of 8 weeks of treatment with either vitamin D₂ at 25,000 IU for 4 days a week alone or in combination with ramipril (0.5 mg·kg⁻¹) on aortic valve structure and function were examined in New Zealand white rabbits. Echocardiographic aortic valve backscatter (AV(BS)) and aortic valve:outflow tract flow velocity ratio were utilized to quantify changes in valve structure and function.

KEY RESULTS

Treatment with ramipril significantly reduced AV(BS) and improved aortic valve :outflow tract flow velocity ratio. The intravalvular content of the pro-oxidant thioredoxin-interacting protein was decreased significantly with ramipril treatment. Endothelial function, as measured by asymmetric dimethylarginine concentrations and vascular responses to ACh, was improved significantly with ramipril treatment.

CONCLUSIONS AND IMPLICATIONS

Ramipril retards the development of AVS, reduces valvular thioredoxin-interacting protein accumulation and limits endothelial dysfunction in this animal model. These findings provide important insights into the mechanisms of AVS development and an impetus for future human studies of AVS retardation using an angiotensin-converting enzyme inhibitor.

Aortic valve disease and treatment: the need for naturally engineered solutions

The aortic valve regulates unidirectional flow of oxygenated blood to the myocardium and arterial system. The natural anatomical geometry and microstructural complexity ensures biomechanically and hemodynamically efficient function. The compliant cusps are populated with unique cell phenotypes that continually remodel tissue for long-term durability within an extremely demanding mechanical environment. Alteration from normal valve homeostasis arises from genetic and microenvironmental (mechanical) sources, which lead to congenital and/or premature structural degeneration. Aortic valve stenosis pathobiology shares some features of atherosclerosis, but its final calcification endpoint is distinct. Despite its broad and significant clinical significance, very little is known about the mechanisms of normal valve mechanobiology and mechanisms of disease. This is reflected in the paucity of predictive diagnostic tools, early stage interventional strategies, and stagnation in regenerative medicine innovation. Tissue engineering has unique potential for aortic valve disease therapy, but overcoming current design pitfalls will require even more multidisciplinary effort. This review summarizes the latest advancements in aortic valve research and highlights important future directions.

Aortic valve disease: preventable or inevitable?

Calcific aortic valve stenosis is the most frequent valve disease and the most common cause of aortic valve replacement in the western world, concomitant with aging of the general population and habitual consumption of a high-calorie diet. For years it was considered to be a passive wear and tear process but now it is recognized as an active process similar to atherosclerosis with involvement of several mediators, such as adhesion molecules, TGFs, cathepsin enzymes and bone regulatory proteins. As conviction grew that aortic stenosis has a genesis similar to atherosclerosis, the hypothesis that statins might be able to alter the progression of the disease also grew. Various retrospective studies confirmed the benefits of statin use at an earlier stage of the disease, but some disappointing results were demonstrated by randomized clinical trials.