Margaret E. McCormick

Bone marrow-derived human mesenchymal stem cells become quiescent on soft substrates but remain responsive to chemical or mechanical stimuli.

The microenvironment of bone marrow-derived human mesenchymal stem cells (hMSCs) strictly regulates their self-renewal and differentiation. Culturing these cells ex vivo leads to a rapid expansion followed by senescence, which is characterized by a lack of proliferation and differentiation. In this study, 250-Pa polyacrylamide gels, which mimics the elasticity of bone marrow and fat tissues, were coated with a mixture of collagen type 1 and fibronectin. When hMSCs were seeded sparsely on these gels, they halted progression through the cell cycle despite the presence of serum, but when presented with a stiff substrate, these non-proliferative cells reentered the cell cycle. Non-proliferative hMSCs on 250-Pa gels also exhibited the capability to differentiate into adipocytes when cultured in adipogenic induction medium or into osteoblasts if transferred to a stiff substrate and incubated with osteoblast induction medium. These results demonstrate that hMSCs on 250-Pa gels are quiescent but competent to resume proliferation or initiate terminal differentiation with appropriate cues. These observations suggest that mechanical signals from the elasticity of the extracellular matrix may be one of the factors that enable the bone marrow niche to maintain MSCs as a reservoir for a long period.

Extracellular Gelsolin Binds Lipoteichoic Acid and Modulates Cellular Response to Proinflammatory Bacterial Wall Components

The various functions of gelsolin in extracellular compartments are not yet clearly defined but include actin scavenging and antiinflammatory effects. Gelsolin was recently reported to bind endotoxin (LPS) from various Gram-negative bacteria with high affinity. In this study we investigate whether gelsolin also interacts with bacterial wall molecules of Gram-positive bacteria such as lipoteichoic acid (LTA) and whether gelsolin’s interaction with bacterial lipids from Gram-negative or Gram-positive bacteria affects their cellular inflammatory responses. A peptide based on the PPI binding site of gelsolin (160–169) binds purified LTA at the same molecular ratio that it binds phosphatidylinositol 4,5-bisphosphate. The OD of recombinant human plasma gelsolin was found to decrease following the addition of purified LTA, and the binding of gelsolin to LTA inhibits F-actin depolymerization by gelsolin. Simultaneously, the ability of LTA to activate translocation of NF-κB, E-selectin expression, and adhesion of neutrophils to LTA-treated human aortic endothelial cells was compromised by gelsolin. Gelsolin was able to partially inhibit LPS- or LTA-induced release of IL-8 from human neutrophils but was unable to prevent Gram-positive Bacillus subtilis or Gram-negative Pseudomonas aeruginosa growth and had no effect on the antibacterial activity of the cathelicidin-derived antibacterial peptide LL37. These data suggest that extracellular gelsolin is involved in the host immune recognition of LTA or LPS following release of these molecules from the bacterial outer membrane during cell division or attack by drugs and immune components.

Spatial phenotyping of the endocardial endothelium as a function of intracardiac hemodynamic shear stress

Despite substantial evidence for the central role of hemodynamic shear stress in the functional integrity of vascular endothelial cells, hemodynamic and molecular regulation of the endocardial endothelium lining the heart chambers remains understudied. We propose that regional differences in intracardiac hemodynamics influence differential endocardial gene expression leading to phenotypic heterogeneity of this cell layer. Measurement of intracardiac hemodynamics was performed using 4-dimensional flow MRI in healthy humans (n=8) and pigs (n=5). Local wall shear stress (WSS) and oscillatory shear indices (OSI) were calculated in three distinct regions of the LV - base, mid-ventricle (midV), and apex. In both the humans and pigs, WSS values were significantly lower in the apex and midV relative to the base. Additionally, both the apex and midV had greater oscillatory shear indices (OSI) than the base. To investigate regional phenotype, endocardial endothelial cells (EEC) were isolated from an additional 8 pigs and RNA sequencing was performed. A false discovery rate of 0.10 identified 1051 differentially expressed genes between the base and apex, and 321 between base and midV. Pathway analyses revealed apical upregulation of genes associated with translation initiation. Furthermore, tissue factor pathway inhibitor (TFPI; mean 50-fold) and prostacyclin synthase (PTGIS; 5-fold), genes prominently associated with antithrombotic protection, were consistently upregulated in LV apex. These spatio-temporal WSS values in defined regions of the left ventricle link local hemodynamics to regional heterogeneity in endocardial gene expression.

Integrated Regional Cardiac Hemodynamic Imaging and RNA Sequencing Reveal Corresponding Heterogeneity of Ventricular Wall Shear Stress and Endocardial Transcriptome

Background Unlike arteries, in which regionally distinct hemodynamics are associated with phenotypic heterogeneity, the relationships between endocardial endothelial cell phenotype and intraventricular flow remain largely unexplored. We investigated regional differences in left ventricular wall shear stress and their association with endocardial endothelial cell gene expression. Methods and Results Local wall shear stress was calculated from 4‐dimensional flow magnetic resonance imaging in 3 distinct regions of human (n=8) and pig (n=5) left ventricle: base, adjacent to the outflow tract; midventricle; and apex. In both species, wall shear stress values were significantly lower in the apex and midventricle relative to the base; oscillatory shear index was elevated in the apex. RNA sequencing of the endocardial endothelial cell transcriptome in pig left ventricle (n=8) at a false discovery rate ≤10% identified 1051 genes differentially expressed between the base and the apex and 327 between the base and the midventricle; no differentially expressed genes were detected at this false discovery rate between the apex and the midventricle. Enrichment analyses identified apical upregulation of genes associated with translation initiation including mammalian target of rapamycin, and eukaryotic initiation factor 2 signaling. Genes of mitochondrial dysfunction and oxidative phosphorylation were also consistently upregulated in the left ventricular apex, as were tissue factor pathway inhibitor (mean 50‐fold) and prostacyclin synthase (5‐fold)—genes prominently associated with antithrombotic protection. Conclusions We report the first spatiotemporal measurements of wall shear stress within the left ventricle and linked regional hemodynamics to heterogeneity in ventricular endothelial gene expression, most notably to translation initiation and anticoagulation properties in the left ventricular apex, in which oscillatory shear index is increased and wall shear stress is decreased.

Pulling on my heartstrings: Mechanotransduction in cardiac development and function

Purpose of reviewEndothelial cells line the surface of the cardiovascular system and display a large degree of heterogeneity due to developmental origin and location. Despite this heterogeneity, all endothelial cells are exposed to wall shear stress (WSS) imparted by the frictional force of flowing blood, which plays an important role in determining the endothelial cell phenotype. Although the effects of WSS have been greatly studied in vascular endothelial cells, less is known about the role of WSS in regulating cardiac function and cardiac endothelial cells. Recent findingsRecent advances in genetic and imaging technologies have enabled a more thorough investigation of cardiac hemodynamics. Using developmental models, shear stress sensing by endocardial endothelial cells has been shown to play an integral role in proper cardiac development including morphogenesis and formation of the conduction system. In the adult, less is known about hemodynamics and endocardial endothelial cells, but a clear role for WSS in the development of coronary and valvular disease is increasingly appreciated. SummaryFuture research will further elucidate a role for WSS in the developing and adult heart, and understanding this dynamic relationship may represent a potential therapeutic target for the treatment of cardiomyopathies.