Macromolecular transport within heart valves

Valvular Endothelial Cell Response to the Mechanical Environment-A Review

Heart valve leaflets are complex structures containing valve endothelial cells, interstitial cells, and extracellular matrix. Heart valve endothelial cells sense mechanical stimuli, and communicate amongst themselves and the surrounding cells and extracellular matrix to maintain tissue homeostasis. In the presence of abnormal mechanical stimuli, endothelial cell communication is triggered in defense and such processes may eventually lead to cardiac disease progression. This review focuses on the role of mechanical stimuli on heart valve endothelial surfaces-from heart valve development and maintenance of tissue integrity to disease progression with related signal pathways involved in this process.

Modeling Impaired Coaptation Effects on Mitral Leaflet Homeostasis Using a Flow-Culture Bioreactor

BACKGROUND

Mitral valve (MV) regurgitation constitutes an increasing burden of adult and pediatric cardiac disease tending to worsen over time. Whether altered mechanical forces on leaflets cause valve disease is unknown. Here we show that MV leaflet coaptive strain disruption alters expression of genes critical to leaflet homeostasis.

METHODS

We used a flow-culture bioreactor of rat MVs with flow-induced cyclic coaptation (cycling valve group; n = 4) or in a sustained open state (open valve group; n = 4). After 3 days of culture, leaflet RNA expression was profiled.

RESULTS

More than 48 genes exhibited markedly changed expression when coaptive leaflet strain was disrupted for 3 days (change >fourfold; p < 0.05; cycling vs open valves). Genes exhibiting highly altered expression included Angpt2, Vegf, Cd74, RT1-Da (HLA-DRA), and Igfbp3. Pathway analysis indicated the most significant signaling pathways regulating the expression changes were Hif1α and Tnfα when MV closure was disrupted.

CONCLUSIONS

Disruption of normal MV coaptive strain markedly alters the expression of leaflet genes, demonstrating that cyclic strain is critically important to leaflet homeostasis. We demonstrate a pattern of MV gene expression changes in which hypoxia signaling is prominently increased in response to disrupted strain cycles. Coaptive strain regulation of MV leaflet homeostasis implicates altered strain as a mechanism potentially initiating valve disease. Early repair may prevent progression of disease driven by altered coaptation.

Effect of Side-Specific Valvular Shear Stress on the Content of Extracellular Matrix in Aortic Valves

Responses of valve endothelial cells (VECs) to shear stresses are important for the regulation of valve durability. However, the effect of flow patterns subjected to VECs on the opposite surfaces of the valves on the production of extracellular matrix (ECM) has not yet been investigated. This study aims to investigate the response of side-specific flow patterns, in terms of ECM synthesis and/or degradation in porcine aortic valves. Aortic and ventricular sides of aortic valve leaflets were exposed to oscillatory and laminar flow generated by a Cone-and-Plate machine for 48 h. The amount of collagen, GAGs and elastin was quantified and compared to samples collected from the same leaflets without exposing to flow. The results demonstrated that flow is important to maintain the amount of GAGs and elastin in the valve, as compared to the effect of static conditions. Particularly, the laminar waveform plays a crucial role on the modulation of elastin in side-independent manner. Furthermore, the ability of oscillatory flow on the aortic surface to increase the amount of collagen and GAGs cannot be replicated by exposure of an identical flow pattern on the ventricular side of the valve. Side-specific responses to the particular patterns of flow are important to the regulation of ECM components. Such understanding is imperative to the creation of tissue-engineered heart valves that must be created from the “appropriate” cells that can replicate the functions of the native VECs to regulate the different constituents of ECM.

A theory for water and macromolecular transport in the pulmonary artery wall with a detailed comparison to the aorta

The pulmonary artery (PA) wall, which has much higher hydraulic conductivity and albumin void space and approximately one-sixth the normal transmural pressure of systemic arteries (e.g, aorta, carotid arteries), is rarely atherosclerotic, except under pulmonary hypertension. This study constructs a detailed, two-dimensional, wall-structure-based filtration and macromolecular transport model for the PA to investigate differences in prelesion transport processes between the disease-susceptible aorta and the relatively resistant PA. The PA and aorta models are similar in wall structure, but very different in parameter values, many of which have been measured (and therefore modified) since the original aorta model of Huang et al. (23). Both PA and aortic model simulations fit experimental data on transwall LDL concentration profiles and on the growth of isolated endothelial (horseradish peroxidase) tracer spots with circulation time very well. They reveal that lipid entering the aorta attains a much higher intima than media concentration but distributes better between these regions in the PA than aorta and that tracer in both regions contributes to observed tracer spots. Solutions show why both the overall transmural water flow and spot growth rates are similar in these vessels despite very different material transport parameters. Since early lipid accumulation occurs in the subendothelial intima and since (matrix binding) reaction kinetics depend on reactant concentrations, the lower intima lipid concentrations in the PA vs. aorta likely lead to slower accumulation of bound lipid in the PA. These findings may be relevant to understanding the different atherosusceptibilities of these vessels.

Inflammatory regulation of valvular remodeling: the good(?), the bad, and the ugly

Heart valve disease is unique in that it affects both the very young and very old, and does not discriminate by financial affluence, social stratus, or global location. Research over the past decade has transformed our understanding of heart valve cell biology, yet still more remains unclear regarding how these cells respond and adapt to their local microenvironment. Recent studies have identified inflammatory signaling at nearly every point in the life cycle of heart valves, yet its role at each stage is unclear. While the vast majority of evidence points to inflammation as mediating pathological valve remodeling and eventual destruction, some studies suggest inflammation may provide key signals guiding transient adaptive remodeling. Though the mechanisms are far from clear, inflammatory signaling may be a previously unrecognized ally in the quest for controlled rapid tissue remodeling, a key requirement for regenerative medicine approaches for heart valve disease. This paper summarizes the current state of knowledge regarding inflammatory mediation of heart valve remodeling and suggests key questions moving forward.

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.

Evaluation of the Scottsdale Loop 101 automated speed enforcement demonstration program

Speeding is recognized as a major contributing factor in traffic crashes. In order to reduce speed-related crashes, the city of Scottsdale, Arizona implemented the first fixed-camera photo speed enforcement program (SEP) on a limited access freeway in the US. The 9-month demonstration program spanning from January 2006 to October 2006 was implemented on a 6.5 mile urban freeway segment of Arizona State Route 101 running through Scottsdale. This paper presents the results of a comprehensive analysis of the impact of the SEP on speeding behavior, crashes, and the economic impact of crashes. The impact on speeding behavior was estimated using generalized least square estimation, in which the observed speeds and the speeding frequencies during the program period were compared to those during other periods. The impact of the SEP on crashes was estimated using 3 evaluation methods: a before-and-after (BA) analysis using a comparison group, a BA analysis with traffic flow correction, and an empirical Bayes BA analysis with time-variant safety. The analysis results reveal that speeding detection frequencies (speeds> or =76 mph) increased by a factor of 10.5 after the SEP was (temporarily) terminated. Average speeds in the enforcement zone were reduced by about 9 mph when the SEP was implemented, after accounting for the influence of traffic flow. All crash types were reduced except rear-end crashes, although the estimated magnitude of impact varies across estimation methods (and their corresponding assumptions). When considering Arizona-specific crash related injury costs, the SEP is estimated to yield about $17 million in annual safety benefits.

Valvular endothelial cells and the mechanoregulation of valvular pathology

Endothelial cells are critical mediators of haemodynamic forces and as such are important foci for initiation of vascular pathology. Valvular leaflets are also lined with endothelial cells, though a similar role in mechanosensing has not been demonstrated. Recent evidence has shown that valvular endothelial cells respond morphologically to shear stress, and several studies have implicated valvular endothelial dysfunction in the pathogenesis of disease. This review seeks to combine what is known about vascular and valvular haemodynamics, endothelial response to mechanical stimuli and the pathogenesis of valvular diseases to form a hypothesis as to how mechanical stimuli can initiate valvular endothelial dysfunction and disease progression. From this analysis, it appears that inflow surface-related bacterial/thrombotic vegetative endocarditis is a high shear-driven endothelial denudation phenomenon, while the outflow surface with its related calcific/atherosclerotic degeneration is a low/oscillatory shear-driven endothelial activation phenomenon. Further understanding of these mechanisms may help lead to earlier diagnostic tools and therapeutic strategies.

Macromolecular transport in heart valves. II. Theoretical models

This paper proposes a new, two-dimensional convection-diffusion model for macromolecular transport in heart valves based on horseradish peroxidase (HRP) experiments on rats presented in the first of the papers in this series (Part I; Zeng Z, Yin Y, Huang AL, Jan KM, Rumschitzki DS. Am J Physiol Heart Circ Physiol 292: H2664–H2670, 2007). Experiments require two valvular intimae, one underneath each endothelium. Tompkins et al. (Tompkins RG, Schnitzer JJ, Yarmush ML. Circ Res 64: 1213–1223, 1989) found large variations in shape and magnitude in transvalvular125I-labeled low-density lipoprotein (LDL) profiles from identical experiments on four squirrel monkeys. Their one-dimensional, uniform-medium diffusion-only model fit three parameters independently for each profile; data variability resulted in large parameter spreads. Our theory aims to explain their data with one parameter set. It uses measured parameters and some aortic values but fits the endothelial mass transfer coefficient ( ka= kv= 1.63 × 10−8cm/s, where subscripts a and v indicate aortic aspect and ventricular aspect, respectively) and middle layer permeability ( K[Formula: see text]= 2.28 × 10−16cm2) and LDL diffusion coefficient [ D2(LDL) = 5.93 × 10−9cm2/s], using one of Tompkins et al.'s profiles, and fixes them throughout. It accurately predicts Part I's rapid localized HRP leakage spot growth rate in rat leaflets that results from the intima's much sparser structure, dictating its far larger transport parameters [ K[Formula: see text]= 1.10 × 10−12cm2, D1(LDL/HRP) = 1.02/4.09 × 10−7cm2/s] than the middle layer. This contrasts with large arteries with similarly large HRP spots, since the valve has no internal elastic lamina. The model quantitatively explains all of Tompkins et al.'s monkey profiles with these same parameters. Different numbers and locations of isolated macromolecular leaks on both aspects and different section-leak(s) distances yield all profiles.

Macromolecular transport in heart valves. III. Experiment and theory for the size distribution of extracellular liposomes in hyperlipidemic rabbits

The heart valve leaflets of 29-day cholesterol-fed rabbits were examined by ultrarapid freezing without conventional chemical fixation/processing, followed by rotary shadow freeze-etching. This procedure images the leaflets' subendothelial extracellular matrix in extraordinary detail, and extracellular lipid liposomes, from 23 to 220 nm in diameter, clearly appear there. These liposomes are linked to matrix filaments and appear in clusters. Their size distribution shows 60.7% with diameters 23–69 nm, 31.7% between 70 and 119 nm, 7.3% between 120 and 169 nm, and 0.3% between 170 and 220 nm (superlarge) and suggests that smaller liposomes can fuse into larger ones. We couple our model from Part II of this series (Zeng Z, Yin Y, Jan KM, Rumschitzki DS. Am J Physiol Heart Circ Physiol 292: H2671–H2686, 2007) for lipid transport into the leaflet to the nucleation-polymerization model hierarchy for liposome formation proposed originally for aortic liposomes to predict liposome formation/growth in heart valves. Simulations show that the simplest such model cannot account for the observed size distribution. However, modifying this model by including liposome fusing/merging, using parameters determined from aortic liposomes, leads to predicted size distributions in excellent agreement with our valve data. Evolutions of both the liposome size distribution and total liposome mass suggest that fusing becomes significant only after 2 wk of high lumen cholesterol. Inclusion of phagocytosis by macrophages limits the otherwise monotonically increasing total liposome mass, while keeping the excellent fit of the liposome size distribution to the data.

Macromolecular transport in heart valves. I. Studies of rat valves with horseradish peroxidase

The present study aims to experimentally elucidate subtle structural features of the rat valve leaflet and the related nature of macromolecular transport across its endothelium and in its subendothelial space, information necessary to construct a rational theoretical model that can explain observation. After intravenous injection of horseradish peroxidase (HRP), we perfusion-fixed the aortic valve of normal Sprague-Dawley rats and found under light microscopy that HRP leaked through the leaflet's endothelium at very few localized brown spots, rather than uniformly. These spots grew nearly as rapidly with HRP circulation time before euthanasia as aortic spots, particularly when the time axis only included the time the valve was closed. These results suggest that macromolecular transport in heart valves depends not only on the direction normal to, but also parallel to, the endothelial surface and that convection, as well as molecular diffusion, plays an important role in macromolecular transport in heart valves. Transmission electron microscopy of traverse leaflet sections after 4-min HRP circulation showed a very thin (∼150 nm), sparse layer immediately beneath the endothelium where the HRP concentration was much higher than that in the matrix below it. Nievelstein-Post et al.'s (Nievelstein-Post P, Mottino G, Fogelman A, Frank J. Arterioscler Thromb 14: 1151–1161, 1994) ultrarapid freezing/rotary shadow etching of the normal rabbit valve's subendothelial space supports the existence of this very thin, very sparse “valvular subendothelial intima,” in analogy to the vascular subendothelial intima.

Oxidized low-density lipoprotein is localized in the ventricles of hearts from patients with coronary heart disease

The present study was designed to investigate whether oxidized low-density lipoprotein is accumulated in the left and right ventricular walls of patients with coronary heart disease (n=10) compared with patients with dilated cardiomyopathy (n=9) or healthy heart donors (controls, n=5). Sections from both ventricles of explanted hearts and coronary arteries of the same patients were analyzed by semiquantitative immunohistochemistry for the presence of oxidized low-density lipoprotein. Oxidized low-density lipoprotein was enriched in the left and right ventricular walls from coronary heart disease patients compared with patients with dilated cardiomyopathy (P=0.0012 for left ventricle and P=0.103 for right ventricle) or controls (P=0.0012 for the left ventricle and P<0.05 for the right ventricle). The accumulation of oxidized low-density lipoprotein was higher in the left than in the right ventricles in all three groups. Positive immunoreactivity for oxidized low-density lipoprotein was mainly identified in the endocardium and the subendocardial areas of the ventricles and co-localized with macrophages. Accumulation of oxidized low-density lipoprotein in the ventricles significantly correlated with the enrichment in the respective coronary arteries, whereas only poor correlations were observed between various hemodynamic parameters and ventricular oxidized low-density lipoprotein accumulation. Ventricular accumulation of oxidized low-density lipoprotein seems to be a generalized pathophysiological process which does not exclusively involve the coronary arteries. Higher oxidative stress in combination with impaired oxygen supply in the endocardium could have favored low-density lipoprotein deposition and oxidation.

The cardiac endothelium: functional morphology, development, and physiology

Cardiac endothelial cells, regardless of whether they are from endocardial or from coronary (micro)vascular origin, directly modulate performance of the subjacent cardiomyocytes, resulting in control of the onset of ventricular relaxation and rapid filling of the heart. This review summarizes major features of the morphology, embryology, and comparative physiology of cardiac endothelial cells as well as the experimental observations on how cardiac endothelial cells affect the mechanical performance of the heart. As for the underlying mechanisms of the interaction between cardiac endothelial cells and cardiomyocytes, two working hypotheses have been postulated over the past years; (1) interaction mediated through a trans-endothelial physicochemical gradient for various ions (active blood-heart barrier), and (2) interaction mediated through the release by the cardiac endothelial cells of various cardioactive substances, eg, nitric oxide, endothelin, and prostacyclin. These two mechanisms may act in concert or in parallel.

Accumulation of lipoprotein fractions and subfractions in the arterial wall, determined in an in vitro perfusion system

A large proportion of a dense subfraction of LDL in plasma is coupled with an increased risk of coronary artery disease, CAD. This may reflect an increased inflow of such LDL subfractions into the intima, since the inflow of lipoproteins is supposed to be inversely related to the size of the particles. In order to evaluate this possibility we used an in vitro perfusion system for aortic intima-media from rabbits with experimental atherosclerosis. The uptake of human VLDL, LDL, HDL and subfractions of LDL (LDL1, 1.019-1.035 and LDL2, 1.035-1.063 g/ml) in lesions and non-involved areas was studied. Our results indicate that particle size is an important factor for the clearance of lipoproteins into the arterial tissue, both for plaques (VLDL 7.6, LDL 25, HDL 58 nl/mg wet wt./h) and in other areas (VLDL 3.8, LDL 4.1, HDL 12 nl/mg wet wt./h). Interestingly, the uptake of LDL2 was as much as 1.5-1.9 times higher than LDL1. This supports the view that an increased lipid load in the arterial wall may be one mechanism behind the association between denser LDL and CAD. Our data also suggest that the difference between LDL uptake in plaque (576 nl/mg wet wt.) and other areas (48 nl/mg wet wt.) not only reflects a rapid clearance but a large distribution volume of the intima (plaque > 60%, non-involved areas 5.7%).

Transfer of low density lipoprotein into the arterial wall and risk of atherosclerosis

The aim of the review is to summarize the present knowledge on determinants of transfer of low density lipoprotein (LDL) into the arterial wall, particularly in relation to the risk of development of atherosclerosis. The flux of LDL into the arterial wall (in moles of LDL per surface area per unit of time) has two major determinants, i.e. the LDL concentration in plasma and the arterial wall permeability. LDL enters the arterial wall as intact particles by vesicular ferrying through endothelial cells and/or by passive sieving through pores in or between endothelial cells. Estimates in vivo of the LDL permeability of a normal arterial wall vary between 5 and 100 nl/cm2/h. In laboratory animals, the regional variation in the arterial wall permeability predicts the pattern of subsequent dietary induced atherosclerosis. Moreover, mechanical or immunological injury of the arterial wall increases the LDL permeability and is accompanied by accelerated development of experimental atherosclerosis. This supports the idea that an increased permeability to LDL, like an increased plasma LDL concentration, increases the risk of atherosclerosis. Hypertension, smoking, genetic predisposition, atherosclerosis, and a small size of LDL may all increase the arterial wall permeability to LDL and in this way increase the risk of accelerated development of atherosclerosis. The hypothesis that atherosclerosis risk can be reduced by improving the barrier function of the arterial wall towards the entry of LDL remains to be investigated; agents which directly modulate the LDL permeability of the arterial wall in vivo await identification.

Imaging infections with antibodies. A quantitative autoradiographic analysis

Radiolabeled IgG has recently been demonstrated to effectively image infections. A potential but unproven mechanism for this localization is the specific binding of IgG to Fc receptors on the surface of inflammatory cells in infections. In an animal model of soft tissue infection, quantitative autoradiography was used to measure 125I-labeled IgG and albumin in tissues with a spatial resolution sufficient to associate these proteins with cellular morphology. Gamma camera images at 24 h localized the infection with target-to-background ratios of 2.2 +/- 0.5 for IgG and 2.3 +/- 1.0 for albumin (mean +/- SD). Using quantitative autoradiography at 1 h post-injection, significantly higher concentrations were found in infected thighs of 2-4% of initial plasma concentrations (CPo) as compared to 0.2-0.3% of CPo in noninfected thighs (P less than 0.05); at 24 h post-injection, higher concentrations (7-8% of CPo) were found in infected thighs. Radiolabeled proteins were not inflammatory cell associated and were localized primarily within the edematous interstitial spaces of the infection.