Hemodynamic shear stress and the endothelium in cardiovascular pathophysiology

Endothelium lining the cardiovascular system is highly sensitive to hemodynamic shear stresses that act at the vessel luminal surface in the direction of blood flow. Physiological variations of shear stress regulate acute changes in vascular diameter and when sustained induce slow, adaptive, structural-wall remodeling. Both processes are endothelium-dependent and are systemically and regionally compromised by hyperlipidemia, hypertension, diabetes and inflammatory disorders. Shear stress spans a range of spatiotemporal scales and contributes to regional and focal heterogeneity of endothelial gene expression, which is important in vascular pathology. Regions of flow disturbances near arterial branches, bifurcations and curvatures result in complex spatiotemporal shear stresses and their characteristics can predict atherosclerosis susceptibility. Changes in local artery geometry during atherogenesis further modify shear stress characteristics at the endothelium. Intravascular devices can also influence flow-mediated endothelial responses. Endothelial flow-induced responses include a cell-signaling repertoire, collectively known as mechanotransduction, that ranges from instantaneous ion fluxes and biochemical pathways to gene and protein expression. A spatially decentralized mechanism of endothelial mechanotransduction is dominant, in which deformation at the cell surface induced by shear stress is transmitted as cytoskeletal tension changes to sites that are mechanically coupled to the cytoskeleton. A single shear stress mechanotransducer is unlikely to exist; rather, mechanotransduction occurs at multiple subcellular locations.

Exploring human/animal intersections: converging lines of evidence in comparative models of aging

At a symposium convened on March 8, 2007 by the Institute on Aging at the University of Pennsylvania, researchers from the University's Schools of Medicine and Veterinary Medicine explored the convergence of aging research emerging from the two schools. Studies in human patients, animal models, and companion animals have revealed different but complementary aspects of the aging process, ranging from fundamental biologic aspects of aging to the treatment of age-related diseases, both experimentally and in clinical practice. Participants concluded that neither animal nor human research alone will provide answers to most questions about the aging process. Instead, an optimal translational research model supports a bidirectional flow of information from animal models to clinical research.

Hypercholesterolemia suppresses inwardly rectifying K+ channels in aortic endothelium in vitro and in vivo

Inwardly rectifying K+ (Kir) channels are responsible for maintaining endothelial membrane potential and play a key role in endothelium-dependent vasorelaxation. In this study, we show that endothelial Kir channels are suppressed by hypercholesterolemic levels of lipoproteins in vitro and by serum hypercholesterolemia in vivo. Specifically, exposing human aortic endothelial cells to acetylated low-density lipoprotein or very low density lipoprotein resulted in a time- and concentration-dependent decrease in Kir current that correlated with the degree of cholesterol loading. The suppression was fully reversible by cholesterol depletion. Furthermore, a decrease in Kir current resulted in depolarization of endothelial membrane potential. Most important, the flow sensitivity of Kir currents was also impaired by cholesterol loading. Specifically, flow-induced increase in Kir current was suppressed by 70%, and flow-induced hyperpolarization was almost completely abrogated. Furthermore, we show that hypercholesterolemia in vivo also strongly suppresses endothelial Kir currents and causes a shift in endothelial membrane potential, as determined by comparing the currents in aortic endothelial cells freshly isolated from healthy or hypercholesterolemic pigs. Therefore, we suggest that suppression of Kir current is one of the important factors in hypercholesterolemia-induced endothelial dysfunction.

Loss of PECAM-1 function impairs alveolarization

The final stage of lung development in humans and rodents occurs principally after birth and involves the partitioning of the large primary saccules into smaller air spaces by the inward protrusion of septae derived from the walls of the saccules. Several observations in animal models implicate angiogenesis as critical to this process of alveolarization, but all anti-angiogenic treatments examined to date have resulted in endothelial cell (EC) death. We therefore targeted the function of platelet endothelial cell adhesion molecule, (PECAM-1), an EC surface molecule that promotes EC migration and has been implicated in in vivo angiogenesis. Administration of an anti-PECAM-1 antibody that inhibits EC migration, but not proliferation or survival in vitro, disrupted normal alveolar septation in neonatal rat pups without reducing EC content. Three-dimensional reconstruction of lungs showed that pups treated with a blocking PECAM-1 antibody had remodeling of more proximal branches resulting in large tubular airways. Subsequent studies in PECAM-1-null mice confirmed that the absence of PECAM-1 impaired murine alveolarization, without affecting EC content, proliferation, or survival. Further, cell migration was reduced in lung endothelial cells isolated from these mice. These data suggest that the loss of PECAM-1 function compromises postnatal lung development and provide evidence that inhibition of EC function, in contrast to a loss of viable EC, inhibits alveolarization.

Shear Stress Biology of the Endothelium

The relationships between blood flow, mechanotransduction, and the localization of arterial lesions can now be advanced by the incorporation of new technologies and the refinement of existing methods in imaging modalities, computational modeling, fluid dynamics, and high throughput genomics and proteomics. When combined with traditional cell and molecular technologies, a powerful palette of investigative approaches is available to address shear stress biology of the endothelium at levels extending from nanoscale subcellular detailed mechanistic responses through to higher organizational levels of regional endothelial phenotypes and heterogeneous vascular beds.

Functional expression of Kir2.x in human aortic endothelial cells: the dominant role of Kir2.2

Inward rectifier K+channels (Kir) are a significant determinant of endothelial cell (EC) membrane potential, which plays an important role in endothelium-dependent vasodilatation. In the present study, several complementary strategies were applied to determine the Kir2 subunit composition of human aortic endothelial cells (HAECs). Expression levels of Kir2.1, Kir2.2, and Kir2.4 mRNA were similar, whereas Kir2.3 mRNA expression was significantly weaker. Western blot analysis showed clear Kir2.1 and Kir2.2 protein expression, but Kir2.3 protein was undetectable. Functional analysis of endothelial inward rectifier K+current ( IK) demonstrated that 1) IKcurrent sensitivity to Ba2+and pH were consistent with currents determined using Kir2.1 and Kir2.2 but not Kir2.3 and Kir2.4, and 2) unitary conductance distributions showed two prominent peaks corresponding to known unitary conductances of Kir2.1 and Kir2.2 channels with a ratio of ∼4:6. When HAECs were transfected with dominant-negative (dn)Kir2.x mutants, endogenous current was reduced ∼50% by dnKir2.1 and ∼85% by dnKir2.2, whereas no significant effect was observed with dnKir2.3 or dnKir2.4. These studies suggest that Kir2.2 and Kir2.1 are primary determinants of endogenous K+conductance in HAECs under resting conditions and that Kir2.2 provides the dominant conductance in these cells.

Endothelial protein kinase C isoform identity and differential activity of PKCzeta in an athero-susceptible region of porcine aorta

Endothelial protein kinase C (PKC) signaling was investigated in different regions of normal porcine aorta. The locations map to differential atherosclerotic susceptibility and correlate with sites of disturbed (DF) or undisturbed (UF) local flow profiles. Endothelial lysates were isolated from the inner curvature of the aortic arch (DF; athero-susceptible) and a nearby UF region of the descending thoracic aorta (UF; athero-protected), and in some experiments a distant athero-protected UF site, the common carotid artery. Total endothelial PKC activity in the DF regions was 145% to 240% of that in both UF locations (P<0.05), whereas the UF regions were not significantly different from each other. PKC protein isoforms alpha, beta, epsilon, iota, lambda, and zeta were expressed in similar proportions in both aortic regions, suggesting that differences of kinase activity were not directly attributable to expression levels. Inhibition of members of the "conventional" and "novel" PKC families had no differential effect on regional kinase activity. However, inhibition of PKCzeta, a member of the "atypical" PKC family, reduced the DF lysate kinase activity to that of UF levels (NS P=0.35). Differential phosphorylation of PKCzeta Thr410 and Thr560, along with increased levels of PKCzeta degradation products in UF endothelial lysates, suggested posttranslational modification of PKCzeta as the basis for site-specific differences in vivo. Steady-state regional heterogeneity of an important family of regulatory proteins in intact arterial endothelium in vivo may link localized athero-susceptibility and the associated hemodynamic environment.

Regional determinants of arterial endothelial phenotype dominate the impact of gender or short-term exposure to a high-fat diet

Regional arterial hemodynamics correlates with distinct endothelial phenotypes that may be modified by risk factors to influence focal and regional susceptibility to atherosclerosis. We compared endothelial transcript profiles from hemodynamically distinct arterial regions in 15 mature pigs: males and females fed a normal diet, and males fed a high-fat diet (15% lard, 1.5% cholesterol) for two weeks. Hierarchical clustering analysis showed preferential grouping of arrays by region over risk factor. A set of differentially expressed genes was identified which clearly distinguished regions of disturbed flow from undisturbed flow; however, few differences were observed within the same region based on gender or diet. Consistent with previous results in the absence of risk factors, the balance in gene expression was not inherently pathological at this early time-point. The results implicate regional hemodynamics as a predominant epigenetic determinant of endothelial phenotypic heterogeneity underlying atherosusceptibility in vivo.

Spatial heterogeneity of endothelial phenotypes correlates with side-specific vulnerability to calcification in normal porcine aortic valves

Calcific aortic valve sclerosis involves inflammatory processes and occurs preferentially on the aortic side of endothelialized valve leaflets. Although the endothelium is recognized to play critical roles in focal vascular sclerosis, the contributions of valvular endothelial phenotypes to aortic valve sclerosis and side-specific susceptibility to calcification are poorly understood. Using RNA amplification and cDNA microarrays, we identified 584 genes as differentially expressed in situ by the endothelium on the aortic side versus ventricular side of normal adult pig aortic valves. These differential transcriptional profiles, representative of the steady state in vivo, identify globally distinct endothelial phenotypes on opposite sides of the aortic valve. Several over-represented biological classifications with putative relevance to endothelial regulation of valvular homeostasis and aortic-side vulnerability to calcification were identified among the differentially expressed genes. Of note, multiple inhibitors of cardiovascular calcification were significantly less expressed by endothelium on the disease-prone aortic side of the valve, suggesting side-specific permissiveness to calcification. However, coexisting putative protective mechanisms were also expressed. Specifically, enhanced antioxidative gene expression and the lack of differential expression of proinflammatory molecules on the aortic side may protect against inflammation and lesion initiation in the normal valve. These data implicate the endothelium in regulating valvular calcification and suggest that spatial heterogeneity of valvular endothelial phenotypes may contribute to the focal susceptibility for lesion development.

A rapid, reliable method to isolate high quality endothelial RNA from small spatially-defined locations

Phenotypic heterogeneity of the endothelium has important implications for cell sourcing for cardiovascular tissue engineered devices and is fundamental to many cardiovascular diseases. A critical first step to identifying genetic regulators associated with particular endothelial phenotypes is reliable isolation of pure RNA from the cell subpopulations of interest. We present here a rapid method for the isolation of endothelial RNA from small spatially-defined locations, illustrated for two sides of the porcine aortic valve. Endothelial cells were retrieved from fresh tissue by freezing them to a glass substrate, from which they were lysed in guanidine thiocyanate buffer for RNA isolation. Valve endothelial cells isolated by this technique stained positively for CD31 and von Willebrand factor, consistent with an endothelial phenotype, with no evidence of contamination by alpha-smooth muscle actin-positive valve interstitial cells or CD45-positive leukocytes. RNA integrity was excellent in 80% of the samples, with over 100 ng of total RNA typically obtained from each side of the valve. This rapid method yields high quality pure endothelial RNA in sufficient quantities for amplification and subsequent use in device-, cell-, and location-specific transcriptional profiling by microarray technologies, and thus facilitates studies of spatial gene regulation.

Role of lateral cell-cell border location and extracellular/transmembrane domains in PECAM/CD31 mechanosensation

Phosphorylation of tyrosine residues on platelet-endothelial cell adhesion molecule-1 (PECAM-1), followed by signal transduction events, has been described in endothelial cells following exposure to hyperosmotic and fluid shear stress. However, it is unclear whether PECAM-1 functions as a primary mechanosensor in this process. Utilizing a PECAM-1-null EC-like cell line, we examined the importance of cellular localization and the extracellular and transmembrane domains in PECAM-1 phosphorylation responses to mechanical stress. Tyrosine phosphorylation of PECAM-1 was stimulated in response to mechanical stress in null cells transfected either with full length PECAM-1 or with PECAM-1 mutants that do not localize to the lateral cell-cell adhesion site and that do not support homophilic binding between PECAM-1 molecules. Furthermore, null cells transfected with a construct that contains the intact cytoplasmic domain of PECAM-1 fused to the extracellular and transmembrane domains of the interleukin-2 receptor also underwent mechanical stress-induced PECAM-1 tyrosine phosphorylation. These findings suggest that mechanosensitive PECAM-1 may lie downstream of a primary mechanosensor that activates a tyrosine kinase.